Measuring Reflectance 
with the Model R200V Reflectometer

  Model R200V


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At the right is the Model R200V as shown on our Reflectometer Web Page.  This instrument is provided with a reflectance standard -- in this case one with reflectance of 99.65% In this image, the protective cover slide is shown in the withdrawn position as required to calibrate used the instrument.  But we're getting ahead of ourselves.  Let's start at the beginning.

The Model R200V should be turned on for about 20 minutes before attempting to use it.  This allows the electronic circuit to become thermally stabilized as required for consistent measurements.

Two initial operations are required before taking reflectance measurements of samples. First, it is necessary to set the "zero" for the instrument. Then the instrument is set to read the reflectance value which the Reflectance Standard provides.

The image at the left shows the instrument with the cover slide closed to protect the reflectance standard housed beneath.  Note that the meter does not read zero because the black anodized finish on the cover slide does reflect some light back into the Reflectometer head. 

In order to set zero, it is necessary to have a surface which reflects no light. The top surface of the instrument even though black anodized actually still reflects a very substantial amount of light -- here shown to be 2.72%

Rather than providing a "Black Sample" -- which is very difficult to maintain, the Model R200V is provided with an aperture to prevent any light from reaching the black anodized surface.

In the image at the right, the operator's hand is shown closing the aperture to block the test beam from leaving the optical head and preventing any of the light from reaching the optical sensor inside. 

With the aperture closed (below) the readout will typically show somewhere around 0.1 to 0.2% before adjusting the zero -- below showing 0.13% 

The operator adjusts the "SET O"



At the right we see the readout showing 0.02% which is actually a pretty good approximation to zero since the last digit is constantly fluctuating +/- 1 digit.

If the Reflectometer has been allowed to come to thermal equilibrium the zero setting should normally persist throughout the day, but it is always a good idea to perform an occasional check to see that nothing has changed, . 

Such zero drift usually varies with ambient conditions such as temperature, but is independent of the ambient lighting conditions, whether the aperture is closed or open.  If the level of drift from zero becomes too large, simply reset zero as described above. 

It is recommended that the instrument be turned on at the beginning of the day and allowed to stabilize at least 20 minutes before taking readings.  The longer the instrument is operating, the more stable the readings become.

Making a reflectance measurement does not require any further manipulation of controls.  Simply place the sample beneath the optical head, as shown below, and take the reading directly from the meter -- in this case 86.67%  It is easy to make 6 to 10 reflectance measurements per minute. 

Occasionally, throughout the day, the operator should go through these procedures to ascertain that both the zero setting and the calibration setting remain constant.  Small variations from either (+/- .02%) do not significantly affect the actual reflectance reading of the test piece.

Graham Optical Systems would be happy to discuss your requirements for an instrument to measure your samples.  We can provide instruments to measure either specular reflectance (as shown here) or diffuse reflectance. 

If you have specific requirements not met by the Model R200V, please contact us.  Specials are usually not very expensive to provide.

Copyright 2015 Graham Optical Systems  All rights reserved                                      This page last updated November 26, 2015