How was the first integrating sphere calibrated? I understand it requires a known luminous flux source but how did they obtain that without a calibriated IS...
Also, for those of you with DIY IS, how a big difference have you noticed in measured values based on the ambient temp? So let's say, you last measured a light at ambient temp of 70F/21C, what do you get at 80F/26.7C or 60F/15.6C?
The lumen is based on the candela which is based on the Watt which is based on the Joule which is based on the Newton. In practice here I think the Joule is just defined as 1 amp over 1 ohm for 1 sec and they probably use a laser. IOW, the units cancel out and they just shoot a laser or led diode of given power into the sphere/target and see how much it registers.
Ah but the question was how would you have calibrated the newly invented photo sensors when they were invented? How would you be able to translate joules into light sensors. Lasers were invented far later than vacuum tube based photo sensors. So back then to be able to measure lumens with a vacuum tube based system you’d have to have a standard reference which I think was the spermaceti candle.
The standard they used before lasers/diodes was black body platinum/gold emitter. And the candle or whatever before that. However, you can see that the arbitrary number they used (683) is a nice around approximation chosen for continuity from the old method more than anything else.