Photometer Testing Conditions

If you're to get a useful answer, we need to know more about your problem.
What kind of photometer is it?
What is your light source?
What range of light intensities are you intending to use?
What wavelengths?
I'm not saying I can answer your questions, but if anyone can, they'll need at least this amount of detail.

As already indicated, you haven't provided much to go on. It may not be necessary to do the testing in a darkroom but it is likely that it would be easier and less error prone. That's about all I can give you based on your info.

If the photocell is sensitive to infared, you may need to maintain a constant temperature. I would make a portable darkroom large enough for the light source and the sensor. Paint the inside with flat black paint. Put the temperature probe close to the sensor (preferably in back so it won't heat up with the light source. have fun with it.

When units for luminance were first quantified, the experiments carried out by scientists of one persuasion or another used a few different methods. One of the discoveries was that luminous intensity decreased with the square of the distance from a point source, which is scientifically and mathematically sound.

Over a short period of time (relative to the life of the filament), for a given current in an incandescent electric light bulb, the temperature of the filament will produce light at a given wavelength and intensity at some distance L. At a distance 2 X L, the intensity will be 1/4 the value of that at L; intensity will be 1/9 at 3L, etc. At a different filament currents, luminous intensities will be different at the same distances, L, 2L, 3L.

Please keep in mind that different wavelengths of light will be produced at each different current and that the sensor response to different wavelengths will affect the measurements you make, and there's no reason to think that there will be a linear relationship.

A good way to come up with your own units for luminous intensity (lux is taken, for example) would be to find the distances at which the luminous intensities are the same for different currents when measured with the particular kind of sensor you use.