Re: Could you test CYA in complete darkness with speedstir l
The light needs to be indirect light and most of it will be coming in from the sides of the tube, not from the bottom. That is, the light needs to be diffuse coming in from all angles into the bottom, side and top of the tube. Plus, the speedstir light is not very bright.
You can get reasonably close indoors if you have a lot of bright lights in the kitchen, for example. Kitchens are often the best lit larger areas in a house. When I turn on both the overall fluorescent lights in the kitchen and the more concentrated halogen bulbs that are over our island where the sink is located, then I get fairly close results to standing outdoors with my back to the sun though it is still consistently a bit higher in CYA reading by roughly 10 ppm. I do not stand directly under a lamp in the kitchen, but in between a pair of the lamps closer to one of them. Another possibly good indoor place is the master bathroom where there may also be good bright lighting.
Outdoor indirect (i.e. shaded) light from a blue sky in the middle of the day is around 10,000-20,000 lux in intensity compared to around 100,000 lux for direct sunlight (reflecting on bright white snow, for example), but holding the tube in front of you cuts down some indirect light so is perhaps in the 5,000-10,000 lux range. Typical lighting in an office is on the order of 500 or so lux. The brighter kitchen and bathroom scenarios I described are on the order of 750-1000 lux overall.
Lux is lumens per square meter so you can roughly calculate the illumination (illuminance) by dividing the lumens of the bulb by the area it is illuminating and more specifically this works well if you have multiple bulbs so can calculate an illuminated area per bulb (ignoring overlap). Divide this number by 2 to account for inefficiencies. In my kitchen, there are four 75W halogen at 1020 lumens each and the rough square area illuminated at counter height is about 9 square feet or 0.836 meters so this is 1020/0.836/2 = 610 lux. The fluorescent lights add to this, but probably less than half so perhaps I'm at around 800 lux.
Though 800 sounds much smaller than 5,000, the visibility for contrast of seeing the black dot against a white background is a logarithmic effect (see
this paper and
this paper). If you have a particularly bright workspace with multiple high lumen lamps, then that will be closer to the outdoor indirect lighting.
