Intensity Binning

Binning schemes.

As with all processes there are tolerances within the manufacture of LEDs, these take the form of variations in: -

Intensity of the light output (mcd) measured at a specific forward current (I_f)

The dominant wavelength (nm) of the light output
The forward voltage (V_f)a cross the LED

LED Manufacturers measure all of these parameters in their production process. However, the critical parameter in most applications is that of the intensity of the light output. During the production process most LED manufacturers sort the LEDs into ‘Bins’ containing LEDs with a light output falling within a specified range.

Intensity: - The concept behind putting the LEDs into specific intensity bins is that the ‘standard’ human eye should not be able to tell the intensity variations between LEDs within a particular ‘Bin’. A typical example of an intensity Bin scheme is shown below: -

Bin #	5T	4U	5U	4V	5V	4W*
Intensity (mcd) @ IF = 20mA	355 - 450	450 - 560	560 - 715	715 - 900	900 - 1125	1125 - 1400

The majority of LED manufacturers, and all YEG’s suppliers will ensure that all LEDs with in Reel or Bag will contain parts of the same intensity bin. It is possible, usually at extra cost to specify that for a certain application all LEDs are delivered within a range of intensity bins. The tighter the requirement ‘Bin’ the higher the cost and there will also longer lead-times associated with a tight bin selections. Manufacturers are usually unable to supply single Bin options.

Manufacturers will have a ‘sweet spot’ where the majority yield of the production output lies. As the product matures this level will change (normally towards the higher intensity). By maintaining close links to their suppliers YEG are able to help with selecting the most cost effective Bin selections.

Dominant Wavelength: - In ‘colour’ LEDs i.e. non-white the LED manufacturers will measure this parameter, to test that their units are within the specified tolerances and the grade will be specified on the LED test data. However, it is not normally possible to deliver specific Dominant Wavelength ‘Bin’ codes.

A typical Colour coding scheme for Green and Yellow LEDs in shown below: -

Colour Codes for LEDs and Displays (TA - 25 Deg C; Tolerance: +/- 1nm)
Group	Dominant Wavelength (nm)
Group	Green		Yellow
	Min	Max	Min	Max
0	556	559
1	559	561	581	584
2	561	563	584	586
3	563	565	586	588
4	565	567	588	590
5	567	569	590	592
6	569	571	592	594
7	571	573	594	597
8	573	575	597	600

In white LEDs, the colour output (temperature) of the LED is a critical parameter and binning schemes are used to define this parameter. This issue is covered more fully in the LED Lighting section of this site Click here to go to the YEG Opto Lighting site.

Forward Voltage (V_f): - Like a normal silicon signal diode when an LED is in forward bias (i.e. On) there will be a voltage developed across it, known as the Forward Voltage or V_f. This value is a variant in the production process and all LED manufactures will measure it as part of their production process to ensure that all devices are within specified tolerances. V_fwill be different depending on the colour and technology used in the manufacture of the LED die. There are various materials used in the manufacture of LEDs a few examples of the variation in V_f are shown below: -

Material Colour Vf Typ

AllnGaN Green (525nm) 3.4v

InGaN Green (525nm) 2.8v

AlGaInP Yellow/Green (570nm) 1.8v

InGaN Blue (470nm) 3.4v

Usually data sheets will give the Typical (Typ) and Maximum (Max) values of V_fwhich will be sufficient for most ‘constant current’ designs. However, in some designs particularly where the LEDs are being driven by constant voltage, variations in V_f can be an issue. In the example below: -

V supply = 3v

I_f (forward current) = 20mA

V_f (typ) = 1.8v (AlGaInP 570nm)

R1 calculated to be 1.2 / 0.02 = 60Ω

However, with the forward voltage at the maximum limit V_f (max) = 2.4v

The forward current (I_f) = 0.6 / 60 = 10mA (i.e. 50% of the driving current)

A 50% drop in If will cause a noticeable change in the light output of an LED.

In addition when driving a large array of LEDs (even in constant current) variations in V_f in can be an issue. Some manufactures do offer V_f bin selections on certain products and YEG can offer design assistance and help select the right product for a particular application.

Intensity Binning

Expertise

YEG Opto