How to build and calibrate a photo-plane light meter

E.M. Kinsman, Rochester, NY


 
 

Being in the professional science photography business, I often get asked to photograph the strangest stuff using some very weird lens combinations.  Lately, I was asked to take some motion pictures of “microscopic animals”.  So I set up the microscope and attached it to a 35-mm motion picture camera – but how do you measure the exposure?

 A typical answer would be to use a standard film plane meter, but such a device would not fit my situation, and I would still have to perform a calibration on the device.

 The answer to my problem was to simply build the type of meter I needed and then calibrate the device.

 There are a number of photosensitive electrical devices manufactured, and used.  I happened to pick up several photodiodes from a local surplus house.  Photodiodes give electricity when exposed to light.  A standard photometer would take the voltage and put it into an analog circuit to convert it to a readable f-number for a certain film speed.  I would be using the meter with 100 ASA film with a set exposure of 1/50th of a second. The majority of photodiodes give off a relatively small voltage of several hundred millivolts. In the old days of analog voltage meters reading this small voltage would be a problem, but modern digital meters will easily read such a voltage.

Two photodiodes on newsprint to show size.
Taken with a Kodak DC290 with a macro lens

Many different photodiodes will work as a light meter, but the silicon based are probably the best with a spectral sensitivity between 200 and 1100 nanometers.  Gallium detectors have a sensitivity between 400 and 1800 nanometers. Both detectors will need a filter to eliminate any infrared radiation.  Luckily most glass is a good absorber of infrared radiation so I.R  will not be a problem.

Calibration

 I took the photodiode and mounted it at the film plane of an old manual exposure camera – an old lens would also work fine.  I pointed the camera at an evenly illuminated 18% gray card and adjusted the light so that the illumination was f5.6 at 1/50th sec with 100 speed film.  I then took readings of the voltage from the photodiode as the lens was changed from f22 to f3.2 and made a graph of the results.  It is important to illuminate the 18% gray card with the same or similar light source as your microscope will use.  I used a 3400 K tungsten source, the same color temperature as my fiber optic microscope illuminator.   The lens in this part becomes the standard - the better the lens the better the results.  If you can find a lens with T stop markings than the results will be slightly better.  T stops are similar to f-stops, but represent the real light change due to an aperture change and not the calculated change as a f-stop does.  I like to use the Microsoft product Excel as a graphing spreadsheet. Once I have entered my data – I can select the data I want to graph and also create a trendline with an equation.

 Once I know how my data fits with a trend line, I can use the equation generated to extrapolate to other f-numbers in between my data points.

 From this data we know the correct exposure and can change it according to the change in film speed or shutter speed.
 

 To calculate a correct exposure for 100 ASA film the equation

 IT = C/S    can be used where

 I = Lux, a unit of light intensity
 C = a constant of 10 when using Lux
 T = Time in seconds
 S = ASA film speed

 Thus 100 ASA film requires 0.1 Lux-seconds to be correctly exposed.

As a side note if you have never calculated f-numbers then there is an interesting relationship. The f-stop or f-number is defined by the equation

 f-stop = (sqrt 2) ^n
 
 Where n is a whole integer 1, 2, 3, etc.

With all these relationships in hand, the calculations becomes quite easy.   Once again I use the program Excel to do the calculations and display the results in a spreadsheet form

Below is my finished calibration for the Lab Notebook, of course your results will depend on your selection of photodiodes.

Before filming I like to verify the calculations with a black and white film test.  This turns out to be a success as shown below.  In this case I used a fixed slide of mixed diatoms.  The diatoms are very good for testing the resolution of the system.

A 35 mm negative scanned

Onward to a color negative film test.  The purpose of this test is to check the color balance of the light source and to really nail down the exposure.  Since this test is done on color reversal (slide film) the exposure latitude is quite tight.  Here you can see the test is also a success, but I would have been better off to use an un-stained plant stem for the test.  This stem is both dyed green and red and the finished film is quite like the slide with an over-all green cast.

A digital picture of the resulting film strip, taken
with a Kodak DC290 with a macro lens

Conclusion

 As you can see by the whole process, it is not very difficult nor is it excessively time consuming.  For whatever photodiode you come by, the process must be repeated.  Build your own meter and start taking those amazing photos!

 By-the-way if you are wondering why I am filming using a motion picture camera?  In motion picture work, the camera films 24 frames a second where each frame has the digital equivalent an 18-megabyte file as you can calculate - this represents an incredible amount of data each second.  Film still seems to be the data collection medium of choice, at least for a few years yet.
 

If you have questions or comments about this article , please do not hesitate to contact me, Comments to the author sent via our contacts page quoting page url plus : ('ekinsman','')">Ted Kinsman, home pages at www.sciencephotography.com .

Reference Texts.

Camera Technology (The Dark Side of the Lens)
Norman Goldberg
Acedemic Press, Inc.  1992
ISBN 0-12-287579-2
 

Photography Through The Microscope
The Kodak workshop Series  1988
John Gustav Delly
Kodak Series #  E1528371  ISBN 0-87985-362-X
 

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