What is the concept of Night Vision Imaging?
The concept of night vision imaging is to intensify available night time near (IR) radiation (600 nm to 930 nm), sufficiently to be presented to the eye on a miniature green phosphor screen. Nighttime infrared radiation comes from such sources as starlight and moonlight.

What is NVG compatibility?
A light source is NVG compatible if it does not interfere with the operation of the goggles. This means that the light has been filtered to suppress the near IR radiation to a level at which the goggles cannot detect its presence, and that it meets specific colour requirements. For more information refer to Application Note AN 1030: LED Displays and Indicators and Night Vision Imaging System Lighting.

Why should enhancement filters be used?
The objective of contrast enhancement is to provide good display readability in the end using ambient light. The concept is to employ both luminous and chrominance (colour) contract techniques to enhance the readability by having the “off” LEDs blend into the background and the “on” LEDs stand out vividly against the same background.

How does a filter improve readability?
The background is illuminated by the ambient light reflecting
off the display surface. Filters work by attenuating the ambient light twice and the LED Hot once. A filter attenuates the ambient light when the light passes through the filter and after the ambient light has reflected off the background. A filter only attenuates light as it passes through the filter. Thus, a filter increases the difference between the “off” LEDs and the “ on” LEDs.

What is candlepower?
Candlepower is an expression that may be used by European Automotive Manufacturers to define intensity. Candlepower is simple luminous intensity expressed in candelas. One candlepower equals 1 candela.

How are colour bin limits determined?
colour bins limits are based on the sensitivity of the eye to changes in the dominant wavelength.

Why are LEDs binned for colour?
LEDs are binned for colour so that the human eye cannot notice the differences in colour due to manufacturing variations.

What affects the Junction Temperature of LEDs?
The following parameter’s affect the junction temperature of LED’s:

1. Ambient Temperature
2. Power Dissipation (Forward current times forward voltage)
3. Thermal resistance, junction to cathode pin (This specification is listed in product data sheet.)
4. Thermal resistance, pin to air For more information review Application Brief AB A04 - LED Lamp Thermal Properties.

What pitfalls are associated with LEDs connected in parallel?
If two or more LEDs with different electrical characteristics (Forward Voltage (Vf) Vs. Forward Current (If)) are connected in parallel, then the resulting voltage across the LEDs connected in parallel will tend to take on the Vf of the LED with the lowest Vf. This is called “ current-hogging” and can cause noticeable brightness differences between LEDs. This issue is detailed in Application Brief AB D 007.

What are the concerns when connecting LEDs in parallel?
The forward voltage of LEDs varies from LED to LED. LED forward voltage increases with an increase in forward current. When LEDs are connected in parallel, the voltage drop across each LED is identical. However, different amounts of current are needed to achieve the same forward voltage drop. Thus, an LED with a lower forward voltage will need more current than an LED with a higher forward voltage. This difference in forward current will cause a difference in intensity. In some cases this difference is objectionable. Therefore, each LED should have its own current limiting resistor.

How do LEDs respond to different temperatures?
In general, the colder a region, the more brightly the LEDs shine. Higher temperatures produce a slightly dimmer effect. This darkening varies from one region to another and has to be scrutinized on the spot. One consideration for the designer when choosing LEDs for a particular application is the way LEDs responds to temperature. In warmer climates and at higher current drive conditions, the LED chip in the lamp tends to become warmer. When an LED gets warmer the colour tends to move slightly toward the red end of the colour spectrum, or longer wavelengths, at about 0.1 nm/degrees C; conversely, the light emitted from a cooler LED tends toward blue or shorter wavelength. LED brightness responds to temperature such that at higher temperatures the intensity is lower. At lower temperatures, LEDs are more efficient, and therefore brighter. Finally, there is a variation in the forward voltage of an LED. As the LED gets warmer, the forward voltage drops slightly, or as the LED cools, the forward voltage rises.

How should current limiting resistors be connected to LEDs (Series or Parallel)?
Using one resistor to limit the current through a string of LEDs connected in series is acceptable. Using one resistor to limit the current through a string of LEDs connected in parallel is not recommended.

Why Can’t We Expand the Storage Temperature Since It Can Be Exceeded for Repotting?
Exceeding the absolute maximum storage temperature will cause the epoxy to yellow very quickly. However, a short, onetime occurrence for reporting is acceptable. Also, expanding the storage temperature range may cause problems with reliability testing of the parts. Specifically, the temperature cycling and power temperature cycle.

What is the UL Rating/Listing of our lamps?
Agilent does not normally submit optoelectronic components to UL(Underwriters Laboratories) for certification. Agilent has deliberately chosen not to have Agilent components tested by UL for the following reasons:

1) Every individual part number has to be separately tested by UL, which is very costly.
2) Every time the material of the part changes, recertification by UL would be required.
3) Most applications require that the end product meet UL listing requirements.

Note: Non-UL listed parts can be used in a customer’s design. However, the overall product has to meet UL requirements.

How is Luminous Efficacy used?
Incandescent light bulb manufacturers use luminous efficacy as an alternative to efficiency. Efficiency is a unitless ratio of input to output. Thus, efficiency for an incandescent light bulb can be calculated by dividing the output radiated radiometric energy in watts, by the electrical input energy. However, this does not give a good indication of how effective the incandescent bulb is at converting energy into visible light. Therefore, they define luminous efficacy as photometric light output in lumens divided by electrical input in watts. LED manufacturers use luminous efficacy to convert photometric lumens to radiometric watts.

This conversion is necessary for all of our customers who don’t use the human eye as their detector, for example, blood analysis equipment, erase bars for copiers, fiber optic communications. An alternate would be to measure LEDs radiometrically. However, it is impractical to have two different measures for intensity. Therefore, because the majority of our business is concerned with the effect of LED light on the eye, we use luminous intensity to measure the light output of our LEDs.

What is the recommended burn-in conditions for our lamps?
There are two methods that our customers can use:
Method 1 - 24 hours @ 50 ma or max drive current of part
Method 2 - 168 hours @ 20 mA or current spec of part
Note: Burn-in is not necessary on our lamps if customer is trying to cause catastrophic failure.

How much does Agilent LEDs weigh?
Here are the weights for various lamp products:

What is the Flammability Rating of Agilent Parts?
Lamps will not spontaneously burn, but they will sustain a flame.

How can the polarity on lamps be determined?
1. The cathode lead is shorter than the anode lead.
2. There is a flat section on the flange of T-13/4 lamps on the cathode side of the lamp.

There are several application notes that offer design steps and operational considerations for pulse drive: Application Note AN 1005, Application Brief AB D010 and Application Brief AB I 024.

How do I mix two or more colours?
In multi-colour display applications or even in simple bi colour LED indicator applications, different coloured light from more than 1 LED are mixed to produce a desired colour. Although the science of colour has not been perfected, there is enough psychophysical evidence for us to construct a method to determine the result of a colour mixture. A technical brief is provided to explain this method.

What is contrast enhancement?
Contrast is the light output difference between the ON elements of the display and the
OFF region of the display.

Why is contrast enhancement needed?
Contrast relates to readability. For example, white characters on black paper has better
readability than gray letters on black paper because of contrast.

What measurement units are used in describing light output?
The most common metric is luminance intensity which is measured in candelas (cd). Luminous intensity tells us how much light flux emitting from the LED is contained in a unit solid angle. Luminance or stearance is sometimes used and is measured in terms of luminous intensity per light emitting area. By using luminance, light measurement becomes independent of light emitting area. Luminance is useful when contrast and visibility need to be considered.

What guidelines are in designing an LED display array?
Application Brief AB D 004 offers guidelines on multiplexing, pulse-width modulation and brightness matching.

Can I increase light output indefinitely by increasing the forward current?
Only up to a certain point. Light output will increase as input current increases. However, heat generation will increase as current increases. The forward current is limited by the amount of heat that can be absorbed by the LED package without affecting performance. Since light output is a function of forward current, it is limited by the heat sinking capabilities of the LED package. Some packages such as through-hole lamps can sink more heat than
chip LEDs resulting in a higher maximum forward current for the through-hole lamps.

What metric do I use to compare light output from two or more LEDs?
Luminous sterance is more appropriate for comparing light output since it is independent of
light emitting area. Intensity is flux per solid angle (candela) while sterance is flux per solid
angle per unit light emitting area (nits). Consider two LED segments A and B with A twice the area of B. If A and B have the same intensity, B will be perceived brighter because it has a larger sterance value.

What are the benefits of LEDs in instrument cluster lighting?
There is an entire application brief on this - Application Brief AB A02.

What are the differences between dominant and Peak wavelength?
Peak wavelength is defined as the single wavelength where the radiometric emission spectrum of the light source reaches its maximum. Dominant wavelength is defined as the wavelength of monochromatic light that has the same apparent colour as the light source.

What are the difference between Luminous Intensity, Luminance and Lumens?
Luminous Intensity (IV) is equal to the amount of luminous flux emitted into a very small solid
angle at a defined angular orientation from the light source. The unit of luminous intensity is the lumen/steradian (lm/sr), or candela (cd).

Luminance or Luminous Sterance (LV) is the luminous intensity per unit area of emitting surface. The unit of Luminance is the candela/meter square (cd/m2).

Luminous Flux is defined as the total luminous energy per unit time emitted by the light source into a sphere (360°) surrounding the light source, where the luminous flux is the
radiant flux multiplied by the human eyes sensitivity. The measurement unit for luminous flux is the lumen (lm).

Where to obtain the data sheet/App notes/Reliability data sheet?
All the product data sheet, application notes and reliability data sheet can be downloaded at
this URL: http://www.semiconductor.agilent.com/view/led/. Please contact Farnell InOne for more information or datasheets.

Why is thermal design important?
Thermal design is important for the following reasons:
1) Ensures that the maximum junction temperature is not exceeded. LEDs may fail at ....excessive temperatures.
2) Allows operation of LED at higher currents to get more optical flux.
3) Minimizes reduction in optical flux of LED due to self-heating.
4) Minimizes colour shift of LED.

What are the catastrophic failure mechanism for LEDs?
The most common cause of failure in LEDs is mechanical overstress. If the temperature of
the package reaches its ‘Glass Transition Temperature’ (Tg), then the epoxy expands rapidly,
stressing many of the internal components of the LED. Wire bond breakage, wire bond
detachment, die attach loss all stem from overheated epoxy or cause a delamination between
the chip and the epoxy. Another failure mechanism is ESD for InGaN devices. Our conventional InGaN LEDs and flip-chip InGaN LEDs are classified as ‘Class 1’
and ‘Class 3’ ESD sensitive respectively. Careful handling and circuit design can reduce
the likelihood of the problems

What is the primary cause of colour shift in InGaN?
The major cause of wavelength shift for InGaN is an increase in current. Please refer to the
product technical data sheet for the rate of wavelength change over forward current.

How to order samples of Agilent’s LED?
Please order the samples via Agilent eSamples. You need to be a registered user with Agilent
Partner Portal in order to have the access to eSamples.http://www.semiconductor.agilent.com

Have you any conversion table from Candela to Lumen or Lux?
There is no conversion table available for candela to lumen. Lumen is a measure of the total
luminous power emitted by the die. We do not sell the die only but die in a package/epoxy.
Therefore with the epoxy we are specially interested in the direction of the light rays. That
is what we measure with cd, visible power per solid angle.

What are the X and Y coordinates of the light chromaticity in the CIE
chromaticity diagram?
The binning of Agilent LEDs are based on dominant wavelength. Therefore, the information of the X & Y coordinates of the light chromaticity in the CIE diagram is not available.

Is there any information on LED eye safety?
We have an application brief on LED eye safety on the web. Please refer to Application Brief
AB I 009. http://www.semiconductor.agilent.com

What is the Definition of Quality?
Quality is the extent to which a particular product satisfies or exceeds the expectations of its
consumer.

What is the Definition of Reliability?
Reliability is defined as the probability that a device will perform its intended function for a specified period of time under stated conditions.

What Does Mean Time Between Failure (MTBF) Mean?
Mean Time Between Failures (MTBF) is defined as 1 divided by the failure rate. If, for
example, there is a system with 948 failures in 106 hours, the average time between failure is 106 hours/948 (=1054 hours). The MTBF figure is not a guaranteed failure-free period. It is a mean value (average) of operating times between failures.

What is the Difference Between Point Typical and 90% Confidence
Level Mean?
If a sample of parts is stressed and an MTBF value is calculated, this is often referred to a point typical MTBF. If many samples were used and MTBFs were calculated, they would not all be the same. There would be a distribution of MTBF. It has been determined that the MTBF follow a chi squared distribution. Knowing the MTBF values are distributed
means that the confidence in any one value can not be that great. To improve the confidence in the point typical MTBF to represent the entire population of MTBFs, a confidence interval is usually calculated. Statistically any distribution can be divided into different regions containing different portions of the population. The 90% confidence interval is the region containing 90% of the distribution (with 5% in each of the 2 tails). The interval can be used as an estimate of what to expect from many other samples (or customer use). The statistical inference made from this calculation is that in 90 out of 100 samples we test, or that the customer uses, the MTBF from that sample would be in the 90% confidence interval that
was calculated from one sample.

What is the Difference Between MTBF and Life?
In MTBF calculations, failures are typically defined as catastrophic. At OED we use the
HTOL (High Temperature Operating Life) test to try to induce failures. We are looking
for catastrophic failures. This test represents one of the worst case conditions for the device under test. In most cases we don’t see failures and assume 1 failure for the purposes of
calculation. Life, on the other hand, for LEDs is most associated with the light output
and degradation. The degradation is not factored into the MTBF.

What Happens When My Customers Exceed the Maximum Ambient Operating Temperature?
The maximum ambient temperature has been determined by qualification tests. Exceeding this limit may cause a significant increase in catastrophic failures or degradation. Initially, light
output degradation will increase. However, continued operation above maximum ambient temperatures may cause catastrophic failures. The farther the limit is exceeded, the faster degradation and catastrophic failures will occur. These failures are due to limitations in the packaging. For lamps the coefficient of thermal expansion changes. This change causes increased stress on the LED wire bonds. Eventually the wire bond breaks.

At high ambient temperatures the optical epoxy can discolour and significantly reduce the light
output. Also, light output degradation increases. Thus, catastrophic and parametric failures will increase when the maximum ambient temperature is exceeded.

What Happens When My Customers Want to Exceed the Maximum Drive Current?
The limit for drive current is determined by the maximum junction temperature and light
output degradation. Initially, exceeding the maximum drive current will increase light output degradation. This occurs even when the maximum junction temperature is not exceeded. Exceeding the maximum junction temperature can cause catastrophic failures.

For a product with a short operating life, exceeding the maximum drive current may be
acceptable. For example, as a rescue buoy, the buoy’s expected life is about 10 operating hours. The life of the buoy is shorter than the parametric failure caused by accelerated degradation.

What is the recommended drive currents for InGaN LED Lamps?
It is recommended that designers use LED drive currents between 10mA and 30mA for best overall long term performance. Drive currents in excess of 30mA do produce higher light output, but with a considerable penalty in high long term light output degradation, and therefore are not recommended.

What type of LEDs are recommended for use in traffic signals?
We recommend our Precision Optical Performance T-1 3/ 4(5mm) InGaN Bluish-Green LED lamps which is specifically designed to meet the needs of the traffic signal market. The
enhancement includes:

1) New encapsulating epoxy which has superior resilience to moisture absorption, contains
uv-a and uv-b inhibitors, a high glass transition temperature and the Tj max is raised to +130 deg Celsius.
2) The internal optics of these lamps have been designed to ensure a precise optical spatial
radiation pattern; the optical axis of each LED lamp is closely aligned with the mechanical axis of the lamp package. Can we support this part number with tape & reel or ammo pack?
Our goal is to consolidate as much demand for taping as possible to ammo pack option.
We can also support tape & reel but on a special basis.

Can we support this part number with tape & reel or ammo pack?
Our goal is to consolidate as much demand for taping as possible to ammo pack option.
We can also support tape & reel but on a special basis.

Under what conditions will we setup part numbers for specific customer’s intensity and colour selection requests?
Depending on the requests, factors include the strategic importance of the customer,revenue potential, yield and ability to sell the fallout. Our goal is to minimize additional part numbers and we believe we have created a competitive product offering.

Where can I get information on soldering though-hole LEDs?
Application Note AN 1027 contains comprehensive information.

Where can I get information on lamp thermal properties?
1. Application Brief AB A04 explains thermal considerations that must be taken into account and thermal design steps.
2. Application Brief AB I 002 contains typical thermal resistance values for lamps.
3. Application Brief AB A05 details how thermal testing is carried out.

How long is the epoxy meniscus?
The epoxy is allowed to descend.040 in.

Where is the LED located within a lamp package?
Typically, a customer who is asking for this information is designing a secondary lens.
Unfortunately, the die location is not precise enough to design a good imaging optics secondary lens. There are several reasons why this occurs.
1. To design a lens the designer needs to know the near field radiation pattern. This pattern is different from the radiation pattern shown in data sheets. Also, we have never characterized the near field radiation pattern of our lamps.
2. The customer is assuming the LED is a point source. This holds when viewed from a
distance. In the near field the LED is a three dimensional light source. Light can be emitted from the top and sides of the die.
Table 1.
3. The relative size between the LED die and the reflector cup. The die is approximately 10 mills by 10 mills, and the reflector cup is significantly larger. The reflector cup size varies for different lamp designs and is considered proprietary. Thus, the location of the LED die within the reflector cup can vary.
4. The insertion depth has a significant impact on the radiation pattern. This is well controlled, but can not be measured after manufacturing. Typical tolerance is about ±0.005 in. the insertion depth has a very significant impact on the lamp radiation pattern.
5. The mechanical axis and optical axis do not always align. Thus, the peak intensity is not always aligned with the optical axis. All of these variations are not measured. However, the method which we use to measure the lamp’s optical performance is on-axis intensity. This measurement is used as an overall control of the manufacturing process. Note: Exterior lighting is an exception. We have an application note which tells how to design secondary optical systems for exterior applications. Exterior applications do not need the precise location of the die.

To calibrate Agilent and our customer’s testers, a request for calibration sample must be
made on a SRF. The actual device and drive current must be provided. This sample can
then be used by our customers to calibrate their tester directly or to create a conversion table
between readings from Agilent testers and their testers.

What is the distribution of parts into colour bins?
This information varies across product lines and is subject to change.

Will my front panel be matched if I match the intensity for the different parts?
No. Displays will appear matched when the sterance for the different displays is matched. Sterance is a good quantitative measure of the subjective perception of brightness. Application Note AN 1031 provides a detailed method of how to match different parts.
For additional information see Application Note AN 1031; or, for an overview, see the section on matching in this document.

Why is intensity measured if sterance is a better measure of the effect light has on the observer?
While sterance is a good measure of the effect light has on the observer, it unfortunately
cannot be measured in a cost effective manner. Also, it is only reliable for diffused parts. It is not effective for direct view displays. The difficulty with direct view displays is trying to
determine the light emitting area. For example, take 0.15", 0.2" and 0.27" SAN displays.
Let’s try using the area of the LED die. This assumption implies that the three SANs
should appear to have the same brightness. However, our eyes tell us that the 0. 15" is brighter than the 0.2" which is brighter than the 0.27". Next, suppose we use the character area. Thus, a ‘ #‘ displayed on 0.1511 will calculate to be brighter than a ‘#’ displayed on a 0.2", which will calculate to be brighter than a’#’ displayed on a 0.27". In this case, the sterance is equal to the sum of the intensities of all the on LEDs divided by the character size area. However, what happens if we compare a ‘ #’ to a ‘.’ on the same character height display? Our eyes tell us that they appear to have the same brightness, but our calculation tells us that the ‘#’should be significantly brighter than the ‘.’. Thus we are unable to determine which area to use when calculating and measuring the sterance of a direct view display.

What is the distribution of parts into the intensity bins?
This information varies across product lines and is subject to change.

Why is the Vr of the flip-chip InGaN LEDs much lower than the Vr of the conventional InGaN LED lamps?
The Vr of the flip-chip InGaN LED package is actually the forward voltage of the protective
diode in the sub-mount. We are currently working on an application brief which explains
the characteristics of our InGaN flip-chip lamps.

How can my customer have a matched front panel?
The best method is to follow the procedure explained in Application Note AN 1031. The eye is good at comparing the relative brightness of two different parts located near each other. Sterance is a good quantitative measure of the subjective perception of brightness. Thus, by matching the sterance of all the parts on the front panel we can achieve a matched panel. Matching occurs when the sterance of the brightest part is within two-tone of the dimmest. Remember that sterance is defined as Luminous Intensity per unit area. Therefore, to match different parts we vary the intensity. We can vary the intensity by changing the LED drive current. Thus, the drive current for each different size and colour display should be adjusted to produce the required intensity. Changing the drive current causes most of the difficulties for our customers. Typically, this happens for one of two reasons. The first is their design has limits on the amount of current allocated for the display. The second is that the circuit is designed to drive all the parts at the same current. Matching parts depends on your customer’s ability to vary the drive current for their application. Selecting intensity bins to match your customer’s design is very risky and can be costly.

Can I have an ultra bright lamp with a wide viewing angle?
No. The amount of flux for a given LED material typically has a tight distribution. This flux can be focused in a narrow beam or diffused into a wide beam. Remember, intensity is incremental flux divided by the incremental solid angle. Thus, a narrow beam will have a high on-axis intensity and a diffused beam will have lower on-axis intensity. Therefore, viewing angle and intensity can be traded. This trade-off is accomplished by varying the insertion depth of the lead frame and/or changing the amount of diffusing material.

Glossary
1... Average Current
......may be broadly defined as the root mean square of the current waveform applied.
2... CIE illuminant C
......6500°K colour temperature source that produces light which simulates the daylight
......produced by an overcast sky.
3... colour Temperature
......temperature of a blackbody radiator whose radiation has the same chromaticity as that
......of a given stimulus.
4... DC forward current
......continuous direct current applied which is constant over time.
5... Dominant Wavelength
......wavelength of the colour spectrum, which, when additively mixed with the light from the ......source CIE illuminant C, will be perceived by the eye as the same colour as is produced ......by the radiated spectrum.
6... Duty Factor
......pulse width divided by the period.
7... Flux
......Rate of energy passing to, from or through a surface or other geometry entity.
8... Frequency
......reciprocal of period, expressed in Hz (1/s).
9... Luminance
......luminous flux per unit solid angle per unit area of emitting surface at an angle with respect ......to surface normal, in candela per square meter or nits.
10. Luminous Intensity
......lumens per steradian (lm/sr) or candela (cd).
11. Nit
......lumens per steradian per square meter, measure of luminance.
12. Peak Current
......maximum instantaneous current that is applied in pulsed operation.
13. Peak Wavelength
......wavelength at the peak of the radiated spectrum.of the radiated spectrum.
14. Period
......time interval from one point to its next/consecutive occurrence in a repeating ......waveform(s).
15. Power Dissipation
......work per unit time consumed by a device, generally the product of current and voltage.
16. Pulse Width
......the interval of device ON time in a period.
17. Radiant Intensity
......watts per steradian (W/sr).
18. Viewing Angle
......typically defined as the included angle which encompasses 50% of maximum intensity ......(sometimes referred to as full width half maximum, FWHM).