The Research Densitometer
The idea for the device was described in a paper by Wooten et al (Investigative Ophthalmology and Visual Science, 40, 2481-2489).
The current machine has the following features:
It is completely calibrated each day (takes only 5 min) so that the measurements are certain. The calibration also allows you to keep track of the normalizing locus at the 7 degree site so that any accumulation of macular pigment outside the fovea can be determined.
A second wavelength at 490nm has been added so that you may verify your optical density at 460 nm if you wish.
We use 4 test targets in addition to the 7 deg reference target. Thus, four optical density values may be determined that refer to .25, .5, 1.0, and 1.75 degrees from the foveal center. (fewer can be made if desired)
We use a psychophysical procedure that keeps the target at constant luminance as the subject varies the ratio of the target radiances. This is an improvement over the standard procedure that we have all used. We have retained the old procedure as an option.
The optics now includes a viewing lens that allows most subjects to run with their distance correction.
Our software calculates optical density, stores the data, fits an exponential function to the data (if you do 4 points), and plots that curve through the data.
A standard DB37 connector allows the user to design a custom computer interface to the instrument.
The new optical system is smaller (8x13x15 inches) (does not require a forehead-chin rest), more portable, and more user friendly.
We are adding to the software a program that allows the calculation, based upon the optical density and the exponential function, of the number of MP molecules or the concentration per square millimeter for any foveal area that you specify. Thus, one could state the MP concentration for a 1 degree target, for example.
We have made more than thirty densitometers for various kinds of investigators including vision researchers, ophthalmologists, epidemiologists, optometrists, nutritionists, and food researchers.
The overall design of the instrument is very conservative. As a consequence they have worked well in the field. Several thousand subjects have been tested. We guarantee all aspects of the device. If there is any problem within 2 years we repair it quickly for no fee. After that we repair it for a nominal fee. Frankly, we have had very good luck. We build the machines to be as trouble free as possible. We take great pride in them and view our customers more as collaborators.
The price (quotation provided upon request) includes a visit from me for 2 full days. During that time I install the machine and instruct laboratory personnel in its use. I do ask for travel expenses.
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