FROM: tooms1490@my-deja.com SUBJECT: Transparency viewer luminance? DATE: Tue, 16 Jan 2001 17:40:13 GMT ORGANIZATION: Deja.com NEWSGROUPS: rec.photo.equipment.large-format,rec.photo.equipment.medium-format,rec.photo.equipment.35mm Light box color temp and CRI standards seem to be fairly common knowledge, but brightness - perhaps an even more critical parameter - is discussed little, if at all.   GretagMacBeth uses 1270 +/-160 cd/m^2  (candelas / meter) as the luminance standard for their transparency viewers (referencing ISO 3664)   Using I (cd/m2) = 2 ^ (EV-3), which I don't have a reference for, this converts to ~13.3 +/- 0.1 EV @ ISO 100   Kodak states it should be 1400 +/- 300 cd/m ^ 2  or  409 +/- 88 foot lamberts  (referencing an ANSI standard, I think)   Using http://www.geocities.com/thombell/charts.html, this converts to   f32-45 1/30 sec @ ISO 400 f16-22 1/30 sec @ ISO 100   Or ~ EV 13.5 @ ISO 100 But is this a proper application of the formulae and charts?   Someone I recently talked to made a distinction between a luminance and reflective meters, stating the former is required for accurately measuring a light box, and that using the latter is a misnomer (measuring light box brightness in EV with a reflective meter).  I've always considered them to be the same, with a reflective meter being able to accurately measure both reflected and emitted light or luminance (w/an incident meter measuring illuminance).   Or is this is more of a semantics discussion? What photogers vs scientists define as luminance?   Can a reflective meter accurately measure light box intensity or luminance, using the standards and conversions above? Thanks, Jim (cross-posted) Sent via Deja.com http://www.deja.com/ FROM: Tom Rittenhouse SUBJECT: Re: Transparency viewer luminance? DATE: Tue, 16 Jan 2001 15:35:17 -0500 ORGANIZATION: Graywolf Photo NEWSGROUPS: rec.photo.equipment.large-format,rec.photo.equipment.medium-format,rec.photo.equipment.35mm tooms1490@my-deja.com wrote: > > Light box color temp and CRI standards seem to be fairly common > knowledge, but brightness - perhaps an even more critical parameter - > is discussed little, if at all. > > GretagMacBeth uses 1270 +/-160 cd/m^2 (candelas / meter) as the > luminance standard for their transparency viewers (referencing ISO > 3664) > > Using I (cd/m2) = 2 ^ (EV-3), which I don't have a reference for, this > converts to ~13.3 +/- 0.1 EV @ ISO 100 > > Kodak states it should be 1400 +/- 300 cd/m ^ 2 or 409 +/- 88 foot > lamberts (referencing an ANSI standard, I think) > > Using http://www.geocities.com/thombell/charts.html, this converts to > f32-45 1/30 sec @ ISO 400 > f16-22 1/30 sec @ ISO 100 > > Or ~ EV 13.5 @ ISO 100 > > But is this a proper application of the formulae and charts? > > Someone I recently talked to made a distinction between a luminance and > reflective meters, stating the former is required for accurately > measuring a light box, and that using the latter is a misnomer > (measuring light box brightness in EV with a reflective meter). I've > always considered them to be the same, with a reflective meter being > able to accurately measure both reflected and emitted light or > luminance (w/an incident meter measuring illuminance). > > Or is this is more of a semantics discussion? What photogers vs > scientists define as luminance? > > Can a reflective meter accurately measure light box intensity or > luminance, using the standards and conversions above? > > Thanks, > Jim If I understand part of your question correctly. No a reflective meter will not read the same as an incident meter. The dome(disk) reduces the light to the 18% standard. To use a reflective meter you would have to reflect the light off a gray card otherwise you get a reading that is about five stops too high. --Tom FROM: dickburk@ix.netcom.com (Richard Knoppow) SUBJECT: Re: Transparency viewer luminance? DATE: Tue, 16 Jan 2001 23:18:17 GMT ORGANIZATION: MindSpring Enterprises NEWSGROUPS: rec.photo.equipment.large-format,rec.photo.equipment.medium-format,rec.photo.equipment.35mm tooms1490@my-deja.com wrote: >Light box color temp and CRI standards seem to be fairly common >knowledge, but brightness - perhaps an even more critical parameter - >is discussed little, if at all. >  >GretagMacBeth uses 1270 +/-160 cd/m^2  (candelas / meter) as the >luminance standard for their transparency viewers (referencing ISO >3664) >  >Using I (cd/m2) = 2 ^ (EV-3), which I don't have a reference for, this >converts to ~13.3 +/- 0.1 EV @ ISO 100 >  >Kodak states it should be 1400 +/- 300 cd/m ^ 2  or  409 +/- 88 foot >lamberts  (referencing an ANSI standard, I think) >  >Using http://www.geocities.com/thombell/charts.html, this converts to   >f32-45 1/30 sec @ ISO 400 >f16-22 1/30 sec @ ISO 100 >  >Or ~ EV 13.5 @ ISO 100 > >But is this a proper application of the formulae and charts? >  >Someone I recently talked to made a distinction between a luminance and >reflective meters, stating the former is required for accurately >measuring a light box, and that using the latter is a misnomer >(measuring light box brightness in EV with a reflective meter).  I've >always considered them to be the same, with a reflective meter being >able to accurately measure both reflected and emitted light or >luminance (w/an incident meter measuring illuminance). >  >Or is this is more of a semantics discussion? What photogers vs >scientists define as luminance? >  >Can a reflective meter accurately measure light box intensity or >luminance, using the standards and conversions above? > >Thanks, >Jim I think you are OK. Candelas and Footlamberts are measures of _luminance_ or photometric brightness, that is of reflected light. The standard appears to measure a diffuse source as though it were a diffuse reflector. Many exposure meters are calibrated directly in photometric units. For instance, the old Weston meter is calibrated in Candles/ft^2 which makes the meter reading independant of any constant which might be in the exposure computer. BTW, 1270 or 1400 cd/m^2 is a very bright source. The conversion factor for cd/f^2 to cd/m^2 is x10.76 --- Richard Knoppow Los Angeles, Ca. dickburk@ix.netcom.com FROM: tooms1490@my-deja.com SUBJECT: Re: Transparency viewer luminance? DATE: Wed, 17 Jan 2001 20:38:49 GMT ORGANIZATION: Deja.com NEWSGROUPS: rec.photo.equipment.large-format,rec.photo.equipment.medium-format,rec.photo.equipment.35mm > The standard appears to measure a diffuse source as though it were a > diffuse reflector. Richard - your sentence below helped clarify things for me. I.E., the standards infer working with a quasi-"Lambertian" surface A reflective meter also seems to respond accurately when measuring the luminance of true light _emitting_ sources - as for my uses, it's essentially just light coming from a surface in a given direction, reflected or emitted. Maybe the accuracy is somehow limited in a strict technical sense or other given applications. Yes - the standards are pretty bright! None of the viewers in my area meet it - all 1/3 to 2/3 EV too dim, including my home unit. This is just past what I'm comfortable with.   Tom - I (unnecessarily) threw in the incident meter comment (re: illuminance, not luminance) Thanks, Jim Sent via Deja.com http://www.deja.com/ FROM: dickburk@ix.netcom.com (Richard Knoppow) SUBJECT: Re: Transparency viewer luminance? DATE: Thu, 18 Jan 2001 01:02:34 GMT ORGANIZATION: MindSpring Enterprises NEWSGROUPS: rec.photo.equipment.large-format,rec.photo.equipment.medium-format,rec.photo.equipment.35mm tooms1490@my-deja.com wrote: >> The standard appears to measure a diffuse source as though it were a >> diffuse reflector. > >Richard - your sentence below helped clarify things for me. I.E., the >standards infer working with a quasi-"Lambertian" surface > >A reflective meter also seems to respond accurately when measuring the >luminance of true light _emitting_ sources - as for my uses, it's >essentially just light coming from a surface in a given direction, >reflected or emitted. Maybe the accuracy is somehow limited in a >strict technical sense or other given applications. > >Yes - the standards are pretty bright! None of the viewers in my area >meet it - all 1/3 to 2/3 EV too dim, including my home unit. This is >just past what I'm comfortable with. >  >Tom - I (unnecessarily) threw in the incident meter comment (re: >illuminance, not luminance) > >Thanks, >Jim Exactly, a true Lambertian surface is assumed. Most (maybe all) light boxes use fluorescent lamps. They are temperature sensitive, the light level increasing with temperature. That means that the full light output is reached after operating for 15 minutes to half an hour, and is somewhat dependant on ambient temperature; fluorescent lamps don't do well in skating rinks. Its possible the error is due to lack of warm up time, or perhaps simply to aging of the lamps. As I said before, this is a pretty bright surface. --- Richard Knoppow Los Angeles, Ca. dickburk@ix.netcom.com FROM: dcalbrecht@aol.com (DCAlbrecht) SUBJECT: Re: Transparency viewer luminance? DATE: 19 Jan 2001 21:09:49 GMT ORGANIZATION: AOL http://www.aol.com NEWSGROUPS: rec.photo.equipment.large-format OK, Now the scoop on this problem. Everyone has mentioned a piece but failed to pull it all together. Maybe this will do it. You cannot comapre FT Lambert/m2 to exposures on a light meter. This is because the specification is for the light output over a square meter (10.75 sq ft). Add it all together and that IS a lot of light. You are comapring apples and oranges. My recommendation, use a reflective meter and measure a new viewer after it has warmed up. Measure it at a number of points around the viewer. If you don't have access to a new viewer ask someone that does (or ask the dealer). From this you will have the starting specification. After that go by the number of hours allowed for operation before replacing the lamps (2500 for most fluorescent viewers, 5000 for Macbeth). This is the only realistic way of evaluating the brightness of a viewer with using a Foot Lambert meter, not something you find at your local phtot store! I have discussed this at length with the Product Line Manager for lighting products at GretagMacbeth. Unfortunately, when I worked there, I never wrote down the values because I could measure them at any time. Regards, Dave Albrecht FROM: dickburk@ix.netcom.com (Richard Knoppow) SUBJECT: Re: Transparency viewer luminance? DATE: Sat, 20 Jan 2001 01:35:29 GMT ORGANIZATION: MindSpring Enterprises NEWSGROUPS: rec.photo.equipment.large-format dcalbrecht@aol.com (DCAlbrecht) wrote: >OK, Now the scoop on this problem. Everyone has mentioned a piece but failed to >pull it all together. Maybe this will do it. > >You cannot comapre FT Lambert/m2 to exposures on a light meter. This is because >the specification is for the light output over a square meter (10.75 sq ft). >Add it all together and that IS a lot of light. You are comapring apples and >oranges. > However, what you are refering to is a statement of energy rather than luminous intensity. An extended true Lambertian surface (i.e., one with perfect diffusion) will measure the same at any distance providing it extends to fill the entrance window of the measuring instrument. That means that a one-foot-square surface will measure exactly the same as one 100 feet square. The meter sees more of the surface as one moves away from it so inverse square law does not have an effect. The specification for the light box is made as though it were a perfectly Lambertian reflecting surface, rather than an extended source. Any meter which is reasonably calibrated in reflected light units should be suitable for making the measurement. The specification for the brightness of color telvision screens is similar, given in Footlamberts or NITs depending on whether its the SMPTE or Sony spec. >My recommendation, use a reflective meter and measure a new viewer after it has >warmed up. Measure it at a number of points around the viewer. If you don't >have access to a new viewer ask someone that does (or ask the dealer). From >this you will have the starting specification. After that go by the number of >hours allowed for operation before replacing the lamps (2500 for most >fluorescent viewers, 5000 for Macbeth). > >This is the only realistic way of evaluating the brightness of a viewer with >using a Foot Lambert meter, not something you find at your local phtot store! I >have discussed this at length with the Product Line Manager for lighting >products at GretagMacbeth. Unfortunately, when I worked there, I never wrote >down the values because I could measure them at any time. > >Regards, > >Dave Albrecht --- Richard Knoppow Los Angeles, Ca. dickburk@ix.netcom.com FROM: tooms1490@my-deja.com SUBJECT: Re: Transparency viewer luminance? DATE: Sun, 21 Jan 2001 06:15:03 GMT ORGANIZATION: Deja.com NEWSGROUPS: rec.photo.equipment.large-format Is there a distinction between photographic light meters, calibrated to measure the "spectral" (?) response of photographic film vs light meters calibrated to measure the "photopic" (?) response of the human eye? Rgds, Jim FROM: dcalbrecht@aol.com (DCAlbrecht) SUBJECT: Re: Transparency viewer luminance? DATE: 25 Jan 2001 19:13:58 GMT ORGANIZATION: AOL http://www.aol.com NEWSGROUPS: rec.photo.equipment.large-format > Is there a distinction between photographic light meters, calibrated to > measure the "spectral" (?) response of photographic film > > vs > > light meters calibrated to measure the "photopic" (?) response of the human > eye? THe question given above is a very good one, and maybe the folks at Minolta, Gossen or Sekonic can be urged to give a response. In theory, they can be quite different. The human eye has an overall response that is most sensitive to green, with red and blue having far less selectivity in some areas, though deep blues are OK for sensitivity. Typical B&W films would be different if for no other reason than the silver emulsion, which is most sensitive or exclusively so to blue light if dye couplers are not included in the emulsion. Panchromatic films do have these dye couplers to give them sensitivity to other colors, but the actual sensitivity through the spectrum is different from one film to another. Sounds like someone can do a bit of research on this. Regards, Have Fun Out There, Dave Albrecht FROM: dickburk@ix.netcom.com (Richard Knoppow) SUBJECT: Re: Transparency viewer luminance? DATE: Thu, 25 Jan 2001 20:28:38 GMT ORGANIZATION: MindSpring Enterprises NEWSGROUPS: rec.photo.equipment.large-format dcalbrecht@aol.com (DCAlbrecht) wrote: >THe question given below is a very good one, and maybe the folks at Minolta, >Gossen or Sekonic can be urged to give a response. In theory, they can be quite >different. The human eye has an overall response that is most sensitive to >green, with red and blue having far less selectivity in some areas, though deep >blues are OK for sensitivity. Typical B&W films would be different if for no >other reason than the silver emulsion, which is most sensitive or exclusively >so to blue light if dye couplers are not included in the emulsion. Panchromatic >films do have these dye couplers to give them sensitivity to other colors, but >the actual sensitivity through the spectrum is different from one film to >another. Sounds like someone can do a bit of research on this. > >Regards, > >Have Fun Out There, > >Dave Albrecht >Is there a distinction between photographic light meters, calibrated to >measure the "spectral" (?) response of photographic film > >vs > >light meters calibrated to measure the "photopic" (?) response of the human >eye? > For the most part photographic light meters do not seem to be filtered to match either film or eye sensitivity. Supposedly the Zone VI meter has filters to match Tri-X (and probably many other Type-B pan films) but most meters just use the raw curve of whatever type of sensor they employ. When Selenium cells were the most often used (non battery meters) separate "tungsten" film speeds were publlished for all films since the Se cells were considerably more sensitive to red light than the film so would yield an underexposure if the daylight speed was used. Cadmium disulfide cells are much closer to film sensitivity so the additional speed rating is not needed with them. Those using Se meters, like the old Weston, GE, Norwood, or modern versions of them might want to use a film speed of about 2/3ds that indicated by the ISO rating when photographing with tungsten light. Curves for many type of cells can be found in the _Handbook of Optics_ published by the Optical Society of America. I've also seen somewhere a graph showing the spectral response of Se cells, Cds cells and the eye, but can't find it at the moment. The calculator in the meter is designed to assume daylight and avarage Type-B pan film and may be incorrect for other types of film or other light sources. Most cameras with built in meters use Cds or sometimes Silicon cells with response fairly close to pan film. --- Richard Knoppow Los Angeles, Ca. dickburk@ix.netcom.com