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     Monopix Tinkerer Username: Monopix Post Number: 128 Registered: 11-2008 Rating: N/A |
Yes, I was going to say the sun as well. Probably the only light source that meets the criteria ("will remain a constant throughout the world within reach of the common man,that is it should not cost so much that we cannot afford it"). Problem with any mains driven light source is that mains electricity varies and so the output wouldn't be constant. Also, two lamps equally rated from different manufacturers won't necessarily give the same light output (the wattage is a power consumption not a light output measure). Incandescent lamps are not usually balanced for daylight either and this can cause different readings with different meters dependant on what type of sensor they are using. I don't think it's an issue though. I always compare meters with a known good one. There are potential problems with this as well like different acceptance angles but I try to compare two cameras of same make with the same lens attached to minimise differences. This also raises the issue of how accurate do we need to be when calibrating meters? I don't think I've ever seen a spec. from a manufacturer as to accuracy so what are we aiming for? I guess most film cameras are used with negative material so slight errors shouldn't be a problem and anyone who uses an old camera with reversal material should be working out their own EI anyway. So, sounds like a good idea but I'm not sure of the benefit. A daylight balanced, variable light source could be useful for checking meters but I don't think it needs to be calibrated - just compare the readings with a known good meter. |