OT: flashlights again

Say more about that please. Doesn't the oil spot test change the amount of radiant power observed?

nearest precise terminology to the common term intensity.

the description of point sources. In the case of

expressed in lumens per steradian = candela.

source is called isotropic. Otherwise, for a flat

of the observation angle with respect to the surface

All true, but the relevant measure of "brightness" for a flashlight would be light reflected from an illuminated target. Seems to me that measuring how perceived brightness increases with increasing power would best be done with the same reflector or collimator (hence same beam angle in steradians), same target and same target distance so the only variable is power.

Reply to
Don Foreman
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I think that's one way it has been done. A group of observers is used and the statistics of the responses are noted.

I've tried side-by-side comparison and I have a hard time deciding when one spot is twice as bright as the other.

I've found it easier to find a point beyond which I may note greater brightness but don't feel the greater brightness is helpful for a given task. That varys a lot with task, of course. Need more brightness to read comfortably than to avoid stepping on the cat or walking into a tree, and so on.

Reply to
Don Foreman

It has been (blush) 40 years since playing the game - in the lab...

Ok - on a card stock cut a nice hole. Across the hole put an oily tissue or such that is stretched tightly.

Now place the two lamps at the end of a 12" or 20" or some length. It needs not be perfect. The card is placed on a carrier that rides on the length of wood or meter stick..... Now in a dark room - look at the oil spot. Total blackness ! - Got You!

Have to light each lamp on the ends with precision voltages (same measured to reasonable accuracy). It can be a battery and a pot that provides a specific VI across and through the diode or lamp and make the other one the same.

Now if one has a 6v lamp and a 12 volt lamp - then make the lamps to their specs.

Once lit, the two lamps have a distance to shine towards each other. The oil spot between. Now with the lights out, look at the oil spot. Do you see the other lamp - through the oil ? move it away from it... until it just goes away. Measure the distance to the oil spot from each lamp.

...

The math is left to the student - but one can easily tell if one lamp is stronger than the other - as it drives further. Inverse square law is law here...

Martin Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH, NRA Life NRA Second Amendment Task Force Charter Founder

D> >

nearest precise terminology to the common term intensity.

the description of point sources. In the case of

expressed in lumens per steradian = candela.

source is called isotropic. Otherwise, for a flat

cosine of the observation angle with respect to the surface

Reply to
Martin H. Eastburn

OK how about this. Dark room and a movie screen where you shine the light at a given distance to make a circle of light a given diameter on the movie screen. Now with a camera on a tripod at a given distance meter the amount of light reflected from the screen. The camera meter will give you a reading in shutter speed and f-stop. Each f-stop and each shutter speed is twice as much light as the next so if the first light metered f8 @ 1/500 sec and the second light metered out f8 at 1/1000 sec. then you would have twice the amount of light.

Reply to
Roger Shoaf

meter stick.....

reasonable accuracy).

the diode or lamp

spot between.

Why not simply use an old photographers light meter? and measure them both, if one wants to be picky..I suppose you could use reflected on a grey card.

Gunner

nearest precise terminology to the common term intensity.

the description of point sources. In the case of

is expressed in lumens per steradian = candela.

source is called isotropic. Otherwise, for a flat

cosine of the observation angle with respect to the surface

Reply to
Gunner Asch

If it helps, each doubling of light energy equates to approximately equal steps of human perception. Photographically speaking, one f/stop corresponds to doubling/halving the exposure time or aperture area. So there it is: human perceptive response is exponential, roughly 2^^n.

Why "old"? They still innovate and make new meters that work on the same (old)principles. Maybe it seems novel now that some don't come with an attached camera. :)

Reply to
Mike Young

"Old" in that few people today buy one, as they did in the "Old Days"

I have about 15 of them...from a very old #2 Wesson to several Sekonics. I use them whenever I drag out the TLRs or the 4x5s, which in the last few years, has been unfortunately rare.

None of the cameras came with metering...least of all the Linhoff.

The some of the Nkons have em..others dont. The S1 has an add on. The Canon A1s of course..shrug..do.

Which I suppose could be used to evaluate the light outputs, if one set a nominal ASA rating, a slow shutter speed, then viewed reflected on a all or grey card...and see what the f-stop differences were.

But a Sekonic might be a better thing..as it will give you candlepower if you so desire.

Hummmm then I suppose one could dig up ones old enlarging meter....

Gunner

"Pax Americana is a philosophy. Hardly an empire. Making sure other people play nice and dont kill each other (and us) off in job lots is hardly empire building, particularly when you give them self determination under "play nice" rules.

Think of it as having your older brother knock the shit out of you for torturing the cat." Gunner

Reply to
Gunner Asch

I have a Toyo. From time to time, I think I'll reinvent the view camera; use the new smaller|lighter|much more expensive linear motion goodies of today, and get rid of the stiction and backlash in the movements.

A spotmeter will probably do a reasonable job with a point light source, although I'll happily send you the cheapie Ilford meter if it surfaces without much digging. It's still pretty new; I only turned it on long enough to plumb its depths and exclaim "What a piece of SH*T!"

Reply to
Mike Young

LOL...I think Ive got one too out in storage, along with the two Omega D2 enlargers..which Ive not used in at least 15 yrs. Someday Ill start up a dark room again. Least Ill think about it and drag my feet and the next of kin will have to deal with the stuff....

Gunner

"Pax Americana is a philosophy. Hardly an empire. Making sure other people play nice and dont kill each other (and us) off in job lots is hardly empire building, particularly when you give them self determination under "play nice" rules.

Think of it as having your older brother knock the shit out of you for torturing the cat." Gunner

Reply to
Gunner Asch

meter stick.....

reasonable accuracy).

the diode or lamp

spot between.

Gottit. This tells you when the luminous intensity from the lamps are

*equal* at the oilspot. It's a lot easier to tell when things are "slightly different" than to tell when one is "twice as bright".
Reply to
Don Foreman

Equal steps, yes, but not necessarily steps of "twice as bright".

It is exponential (or logarithmic, depending on how you look at it), but the base of the logarithm (2 in your example) is not certain.

Photographic film doen't respond in the same way that human vision does -- and different films have very different responses from wide range films like Panatomic X to high-contrast films like Litho.

I think (thin ice here) that there is a factor gamma which is the slope of the exponential response curve.

Reply to
Don Foreman

Dredging the depths of the mostly forgotten arcana, gamma ranges between 1.8 and 2.4. I think the industry settled on 2.2; Macintosh went with 1.8 which was common with graphics arts folks at the time. Film response, of course, can be and almost always is different from perception. For print film, the contrast slope only has match that of the paper. Projected slides, however, don't have that intermediate correction. Even there, the relationships are fungible. Many slide films stretch the slope to improve separation of mid-range tones (at the expense of highlights and shadows), to add "punch" to the image. The image remains believable and not at all objectionable, although it is strongly different from strict reality. This part is important. Our internal definition of "twice as bright" is not fixed. We only need the relationships to remain linear (in a log/exponential sense) to adjust our internal scales so the image makes sense.

I'm not sure where that leaves us. I think 1 f/stop on a photographer's meter is as good a definition us average folks will find for "twice as bright." (I don't expect I'm at all typical, although it doesn't make me any smarter on this subject. I use a photometer from time to time to calibrate my monitor so the tone steps remain linear. Incidentally, it's the white and black points that change as the monitor ages. Linearizing the display only adjusts the spacing of tones within those extremes. The relevance is that it's yet more illustration that our perception is not at all absolute, but changes easily and quietly to suit conditions.)

Reply to
Mike Young

Gunner - that is a D22 I bet. I have a 2 1/4 x 2 1/4 one upstairs now. Got it in

64 when living on island. Last time used it - when in Austin. Have to get it out and stock up on chemicals - those are getting rare.

Martin Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH, NRA Life NRA Second Amendment Task Force Charter Founder

Gunner Asch wrote:

Reply to
Martin H. Eastburn

They are whatever the 4x5 versions are. IRRC its the D2..but Im often wrong.

Gunner

Reply to
Gunner

OH oh - I may have boo booed.

Maybe a 4x5 /2 is a D2/2 or simply D22.

Forgot you had a 4x5 - I once had a beautiful 11x17 battle field view camera. I could not lift the tripod. I think it was 25 feet tall when spread. Kept it in a Storehouse. Left it in the islands - once again attached to the News paper looking for another good home. Once was a U.S. Marine official camera that took the major pictures of Kwajalein Atoll. Most of my shots were with 4x5 and once

8x10. I didn't have developing trays for 11x17 - the film was 'easy' for me to get. Just a call.

Martin

Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH, NRA Life NRA Second Amendment Task Force Charter Founder

Gunner wrote:

Reply to
Martin H. Eastburn

...

For an oil spot technique (as mentioned earlier by someone - Roy or Martin?) see

formatting link
. When the spot (illuminated from opposite sides by two sources) disappears then the brightnesses at the paper surface are judged equal. Assuming inverse square falloff, and source brightnesses a and b, and distances p and q, we have a/p^2 = b/q^2. If a = 2b, then p = q * sqrt(2). Of course, "measured brightness" is not "perceived brightness", and lights perceived as twice as bright will differ from one individual to another and also with color. For example, seeing illuminated red and green blobs of the same size, some people will think the green is larger, and some the red, but most people won't think they are the same size, IIRC.

See the last paragraph of the webpage for oil-spot-test complications such as color temperature and the eye's varying spectral sensitivity. A complication hinted at early in the page (ie, to use "a spherical bulb not a flood light") is that lenses will change the effective distance. If you have a source of strength b known to be omnidirectional, set it at distances q and q', adjust a to match, say at distances p and p', then solve a/(p+r)^2 = b/q^2 and a/(p'+r)^2 = b/q'^2 for r and a. If both sources are directional, take a third measurement at distance q", write 3 equations like a/(p+r)^2 = b/(q+s)^2, etc., and solve for r and s and a.

-jiw

Reply to
James Waldby

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