An Observing Light Standard for Visual Astronomy of the Deep Sky
http://www.clarkvision.com/visastro
http://www.clarkvision.com/visastro
Here is a possible way for everyone with a camera that has a lightmeter to check the brightness of their lights:
1) Use a camera with a 50 mm lens set at f/8, and at ISO 1,000. (lens focal length should not affect the reading significantly.)
2) Place the flashlight right up next to the front of the lens (do not touch the lens--you might scratch it, a few mm away is fine. Shine the light into the camera. In the test I did with my light, the light fills the narrow length of the frame and about 2/3 of the long length, and, of course, it is not in focus.
3) Take an exposure reading in seconds.
I use a flashlight that I've added a reostat to, a 2 cell flashlight with a 4 cell bulb and some paper to make a yellow-orange light. At the level that I use when deep-sky observing, the camera reads:
4 sec f/8 (ISO 1000)
At levels of 1/4 sec, the light is way too bright. If you were to walk around and need ground light (flashlight pointed down, from waist level) I would suspect a level of about 1/10 sec would be dim but ok.
What do you get on your lights? A big question here might be how do camera light meters respond to red light? I think the answer is sensitivity is dropping off, so red readings may be low compared to green-orange.
"Standards--so many to choose from"
Roger Clark
R. N. Clark Email contact (is encoded to prevent spam):
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Last updated Jan 27, 2002