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Below is a Typical 2-channel Transmissometer.  It could also have been configured with one channel as a Transmissometer and one channel as a Densitometer. What are the relative advantages and shortcomings of Transmissometers vs Densitometers?  Let's examine them more carefully.

 2-Channel Model 500W Rack Mount (Shown with Optional Integrator)

When light travels along a given path, some may pass through, some may be absorbed and some may be scattered by dust particles, fog or smoke. Transmissometers measure how much of the light gets through the path without determining whether the "lost" light was absorbed or scattered.  This transmitted portion of the light may be expressed in many ways, two of the commonest being Transmittance (T) and Percent Transmittance (%T). Measurements can be made at one specific wavelengths or in a broader spectral band or even white light which is essentially all visible wavelengths combined.

The Transmittance Scale runs from 0.0 for an optical path that transmits no light to 1.0 for an optical path that transmits all of the light that passes through it.

The Percent Transmittance Scale, (%T) runs from 0.0% for a path that transmits no light to 100.0% for a path that transmits all of the light that passes through it.

Both Transmittance (T) and Percent Transmittance (%T) are measured in the same way, and presenting the measurement as (T) or (%T) is strictly a matter of choice or application.

Well designed Transmissometers produce very linear results over several decades of Transmittance, but for very small transmittance, say less than 1%, small differences can be important and displaying them and interpreting their significance is more challenging.

Densitometers usually begin their lives as Transmissometers and then convert Transmittance to Optical Density in one of two ways.

Using a Logarithmic Amplifier (LogAmp ).The Transmittance measurement can be converted to Optical Density and displayed on a Digital Meter or output to chart recorder or data logger.  This method is relatively inexpensive but can be prone to error due to the LogAmp's sensitivity to temperature change.  It is therefore very important to compensate for variation in temperature if this method of determining OD is chosen.

Using a Computer. The Optical Density (OD) can be calculated from the Transmittance using the mathematical formula:  OD = -log(T).  There ar several options available, one being the use of a Data Acquisition board together with an appropriate software such as National Instruments LabView which rapidly converts Transmittance readings to Optical Density data and can displays both Transmittance and OD data graphically as well as storing data for future access.

The instrument shown at the right is configured to measure both %T and OD at two different wavelengths, one visible and one in the infrared. 

The Chart below produced by LabView shows (T) as a function of Time. 

Note that although full (T) is 1.0, the top of the scale is labeled 1.05 to permit the Operator to set T to exactly 1.0 during calibration.

The graph of T above and the Graph of OD below are simultaneous measurements of the same optical path.  Note that when T drops to a very low level in the Top graph, it is virtually unreadable -- being barely above 0.0 while in the Lower Graph, the OD can clearly be read as averaging around OD = 2.4.  Mathematically this can be converted to T = 0.00398 (i.e. 0.398%T) using the formula
                                                                 T = 10-OD
The inverse formula for converting Transmittance to Optical Density is:
                                                               OD = -logT

For further information about Transmittance and Optical Density please click on the links at the left.

For Further Information, call Gordon Graham today at (818) 700-1263   E-mail
Graham Optical Systems,  9530 Topanga Canyon Blvd.,  Chatsworth, CA 91311
Copyright © 2014 Graham Optical Systems  All Rights Reserved         This page last updated June 9, 2014