# How can I measure the brightness of a LED?

I'm interested in converting voice of the customer (VOC) to LED brightness specification.

In order to do this, I would like to be able to measure LED brightness using a cost effective instrument.

What are the available instruments to measure LED brightness and respective cost effective techniques?

• You could do it with another LED. – Majenko Jan 22 '15 at 23:09
• Not sure if the VOC also includes the specification for reliability, but as LEDs degrade (yes, they do degrade), the wavelength and intensity will change. I've found it is most economical to pursue the LED vendor to determine brightness/wavelength shift over time from the LED vendor, as they have designed it. Osram (although expensive) writes excellent specs for LEDs. - HTH – cowboydan Aug 9 '15 at 21:40

It's not clear if you want to actually encode the voice signal with LED brightness, or whether the brightness is just supposed to indicate recent amplitude. In any case, none of this requires actually measuring LED brightness, so it's not clear what you are really trying to do.

The simplest means to measure brightness is probably with a light dependent resistor (LDR). These are CdS cells that vary quite dramatically with light intensity. The signal is strong, but CdS cells are relatively slow (good for voice amplitude, but borderline for actual voice), and non-linear.

Light can also be measured with photodiodes or phototransistors. These are faster and can be arranged to produce a linear signal. But, they are a little more complicated to drive and their signal is generally "small". They require active amplification to get to typical useful levels, and therefore are subject to picking up noise.

There are two broad ways to use a photodiode, in leakage mode and in solar cell mode. In leakage mode, you reverse bias it. The current is reasonably insensitive to the bias voltage over a wide range, and quite linear with light. It is small though, so is usually run directly into a transimpedance amplifier. In solar cell mode, the diode is kept shorted, and the current resulting from it acting like a solar cell is measured. This produces good results, but usually requires a negative supply to make a positive voltage.

• VOC is a Six Sigma term for customer feedback. – Spehro Pefhany Jan 23 '15 at 1:30

To measure the brightness and color of an LED and get a number that is comparable to the number on the LED datasheet requires a calibrated instrument such as a photometer or spectroradiometer with an integrating sphere or a goniometer (measuring the radiation pattern). Here is one setup.

There is a good introduction to LED metrology here, from German supplier Instrument Systems.

Typically LED brightness is not specified to a very tight tolerance (or required to be specified) but it is good enough for many applications to purchase an LED with a specified range of brightnesses at a given current and verify (from samples) that it looks okay.

If you need to be able to measure it to verify compliance with specifications, I think you'll need an instrument similar to the above-mentioned devices.

You could get a rough comparative measurement (or a measurement of an indirect parameter such as luminous flux per unit area as @trcosley suggests) with other much less expensive means. Lux meters used for measuring illuminance are very inexpensive and commonly available (I have one to measure work area illumination).

If you are literally interested in measuring the brightness of an LED, you could use a light meter like this one.

which is probably a little pricey for your needs but I'm sure you can find others. I included it because I have one and it seems to work well.

It has the following specs:

Range: 400,000 lux; 40,000 fc
Resolution: 0.01 / 1 lux; 0.001/1 fc
Accuracy: ±(3% + 0.5% fs)