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Luxmeter with BPW21R photodiode.

A luxmeter is a device to measure the level of illuminance.
I made a simple luxmeter using a BPW21R photodiode.

Circuit diagram of the luxmeter.

Circuit description:
Photodiode D1 works like a small solar cell, and produces a reverse current when illuminated.
The BPW21R ( datasheet_BPW21R.pdf )  produces a reverse current of 7 nA per lux illuminance.
With switch S1 in position "1 lux/mV", this current goes through (R1 + R2 =) 142 kΩ, and we get 1mV across photodiode D1 for every lux measured.
With the switch in position "100 lux/mV", the current from the photodiode is loaded with (R3 + R4 =) 1.42 kΩ, and the measurement is now 100 lux/mV.
The voltage across D1 can be measured with a digital voltmeter connected to CON 1 and CON 2.
Use a voltmeter with at least 10 MΩ input resistance, to prevent loading the circuit to much.
The circuit works with the current generated by the photodiode, no other power supply is needed.

When a voltmeter is connected to this circuit, the leads of the voltmeter can pick up hum,
capacitor C1 reduces this hum voltage to a low level, so it can't be rectified in D1 and give an error in the measurement.

Note that the photodiode generates a reverse current (that is, current flows from cathode to anode), and this produces a forward voltage across the photodiode (anode gets positive).
We must keep the forward voltage across the photodiode low, to prevent the generated current to leak away through it's own diode conduction.
I found out, the BPW21R can be used up to 300 mV forward voltage without problem.
Therefore the maximum measurements are:
300 lux in the 1 lux/mV setting,
30000 lux in the 100 lux/mV setting.

The luxmeter.

Short circuit reverse current through the BPW21R as function of illuminance.
We get 7 nA current per lux.
Short circuit current will say; the current with zero volt across the photodiode.
In this circuit we don't actually short circuit the photodiode, but the voltage is kept so low (below 300 mV forward voltage) that the short circuit current still applies.

According to the datasheet, the photocurrent increases to 9 nA per lux when there is 5 volt reverse voltage across the BPW21R.


Relative spectral sensitivity vs. wavelength for the BPW21R.

To accurately measure light levels is lux, the measuring device must have the same response to wavelength as the human eye.
The BPW21R has an build in optical filter, which more or less follows this response, but is actually still to wide.
But it is much better the a photodiode without any optical filter.

Example of a photodiode without optical filter, in this case the BPW20R.
The response is totally different then the human eye.
The peak sensitivity is in the invisible IR wavelengths, which makes this photodiode unsuitable for an luxmeter.

The luxmeter in action, here it measures (40.7 mV x 100 lux/mV = ) 4070 lux.
This measurement was done outdoors in the shadow.

The inside of the luxmeter.

To determine if my luxmeter gives accurate readings, I compared it to a commercially available luxmeter, which I borrowed from someone.
The commercial meter was the LX-10 made by Voltcraft, this is a low cost meter.
It's manual says the accuracy in the 0 - 9999 lux range is 4% + 8 digit.

Voltcraft LX-10 luxmeter.

Measurements were done with both meters for several light sources.

Led lamp 1 (cool white) 150 114 0.760 x
Led lamp 2 (neutral white) 110 102.6 0.933 x
Led lamp 3 (warm white) 120 115 0.958 x
Led lamp 4 (warm white) 61 63.6 1.042 x
Led lamp 5 (warm white) 71 73.2 1.031 x
40 Watt incandescent lamp
at 1 metre
33 110 3.333 x
Outdoors (clouded sky) 2800 2940 1.050 x

Measurements with Voltcraft LX-10 and my home-made luxmeter.

For the incandescent lamp there is a large difference in measured values.
Which one is correct?
A 40 Watt incandescent lamp has a luminous flux of about 400 lumen.
When this flux is equally radiated in all directions, the luminous intensity of the lamp will be 400 / 4π = 31.83 candela.
Measured at 1 metre this would give an illuminance of 31.83 lux.
So, the Voltcraft LX-10 result (33 lux) is very close to this theoretical determined value.

The home-made luxmeter gives a much too high lux value for the incandescent lamp.
This has to do with the BPW21R photodiode which is too sensitive for long wavelength.
For instance for 650 nm (red) and longer wavelengths (infrared), where an incandescent lamp emits a lot of energy.

The Voltcraft LX-10 seems to do a better job in filtering out the long wavelenghts.
Also the LX-10 has a maximum measuring range of 199000 lux, this includes direct sunlight (which is up to 100000 lux).
My meter only goes to 30000 lux.
And the LX-10 is only slightly more expensive then the components alone for my home-made luxmeter.

So maybe it's not worth it to build this luxmeter yourself instead of buying one, but it was a nice project to make anyhow.

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