HINTS FOR BUILDING HOMEMADE LUXMETER                                                                                                                          29-8-98
 The light sensor uses a Hamamatsu photodiode, model S1133, which has an active sensitive area of 2.4x2.8 mm with a cyan filter to compensate photodiode spectral response to visible light:320-730 nm with peak sensitivity to 560nm. I recomend ceramic package with 9 mm pins. Sensitive area is located 1 mm beneath filter surface. An alternative,  less expensive, to this photodiode could be  BPW34 -with peak sensitivity to 950 nm- plus a cyan filter to block red and IR light Instead of a single 10Mohm resistor use 10x1M metal film, 20 ppm temperature coefficient. Use a very low leakage 10nF capacitor and an op amp 406 or an equivalent one with very low bias current and stable with temperature. If possible use guard ring around pin 2 and clean solder flux after soldering. Light probe should have a white surface for focussing your negative and photodiode should be located at a level equivalent to your easel/paper thickness. Use a very flexible shielded cable to conect your light probe to your meter.
You probably will have to add some small resistors in series with 10k resistor for your "zero adjust" circuit. Use cermet trimpot for zero adjust. Take care you do not inject digital noise in your analog measuring circuit. Pin 21 -digital ground- should use a "private" return to the power supply. Take care with your power supply capacitor filter currents don't have common returns paths with your signal analog ground. If you want to measure footcandle instead of lux you can adjust your reference voltage Vref to 303 mV instead of 324 mV and shift your display digital points 1 digit right. Sometimes -many times - .47 and .22uF capacitors are not at their nominal capacity and converter could go not linear at high range levels. You can correct this trimming 100k resistor. Use polyester or polycarbonate capacitors only.
 This meter can measure very low light levels and you will need a very stable light source in your enlarger for stable readings. I suggest a voltage stabi lizer for your enlarger bulb if you see your meter never stays quiet!.However you can check overall stability of your meter reading dark current. Put your light probe into a light tight enclosure and make readings during half an hour to check the stability of your electronic system. Once you have a stable light enlarger system you can check light distribution on your easel and real apertures of your lenses. Results probably will shock you!!
For measuring negative densities you can follow these steps:
1-Turn on your enlarger and luxmeter circuit and wait for stable readings
2-Focus negative on your light probe white surface
3-Turn off your darkroom red light
4-Take a reading of your enlarger light source without negative and scan your overall frame for light distribution with the aperture you will use.Minimum illumination on the easel should be 300 mLux. For lower levels open your lens for more light and accurate measurements.If you use your working aperture your density measurements will measure the real density or real illumination arriving to paper independently of how much flare your lenses may have. You don't have to care if your enlarger has condenser or diffuse light source.
5-Take a reading -at a non exposed area of your negative- of your Background+ Fog negative- BF-
6-Interrupt light to your photodiode with any 100% opaque object and take a reading of zero luxmeter- Z-
7-Take readings of your points of interest - R-
8-Go to your computer and compute density D=log((BF-Z)/(R-Z)). Log is base 10

I have written a simple exe file- luxden.exe- for those interested in getting a simple direct density calculator. It only permits to compute 20 density points in a negative with remarks about these points. Sorry it is at an elementary level and does only compute densities! Enlarger light must be more than 300 mLux for computing. Return key instead of a reference input ends this program.
-Francesc Novau