# Transmissive Measurements - How to Measure Spectral, Color and Luminous Transmission

There are generally two steps to transmissive measurements - measuring the reference standard light source with and without a translucent object to collect a baseline. The deviation between these measurements defines the properties of the translucent object.

Image Credit: designmethod/Shutterstock.com

This article details transmissive measurements, before discussing several applications of transmissive measurements. Finally, some basic recommendations are given to ensure precise measurement results.

## Measurement Setup

When precise transmissive measurements are required, a detector and reference standard light source are used to measure transmittance of an object. Figure 1 illustrates a sample setup for transmissive measurements of an object (e.g. red filter). The spectral transmittance, τ(λ), of an object is expressed as the ratio of transmitted spectral flux, Φλt, emitted by a light source and an incident flux, Φλi, which can be measured with a detector.

Figure 1. Transmissive measurement setup with halogen light source and spectrometer to measure an object’s transmission. Image Credit: Admesy

For all translucent objects, some of the light is absorbed, some is reflected, and some is transmitted. The deviation between these three variables is defined by the characteristics of the object.

Based on the conservation of energy, it can be established that the total amount of light emitted by a light source directed towards an object is equal to the sum of absorption α, transmission τ, and reflection ρ of a particular object (equation 1).

The detector can only measure the object’s transmission. It is not possible to measure absorption and reflection using setups and tehniques illustrated in this article - these can only be derived as a sum from the collected data using equation 1.

 (1)

Figure 2. Emitted light is split into absorption, transmission and reflection. Image Credit: Admesy

Based on the object’s spectral properties, various transmissive measurement setups can be used. A colorimeter, spectrometer or light meter is used for X, Y, Z measurements, complete spectral measurements, or Y (luminance) measurements, respectively.

Various light sources can be used to match spectral properties of the object to measure - e.g. a high pass filter for wavelengths <700 nm should not be measured with cool white LED sources due to limited spectral power in the region >700 nm. A halogen source would be more suitable for this application.

## Transmissive Measurement Procedure Using a Spectrometer

An object’s features can be clearly measured using a spectrometer and a broadband reference standard. As not all wavelengths are transmitted through an object equally, it is essential to measure these properties, such as when measuring a material’s UV blocking properties. A spectrometer is capable of accurate measurement of transmittance for each wavelength.

A different light source may be needed depending on the type of object to be measured. For example, a halogen tungsten light source is not appropriate for measuring UV transmittance as the output in this region is very limited, thereby resulting in a poor signal noise-ratio. However, when measuring VIS and NIR, a halogen tungsten light source is ideal.

Figure 3. Overview of transmissive measurement procedure using a Hera spectrometer and halogen light source. Image Credit: Admesy

## Transmissive Measurement Procedure Using a Colorimeter

A colorimeter can be used to establish X, Y and Z values for transmittance based on the required test setup. This setup enables exact measurements for filters with high density factors, when measurements must be performed at low luminance levels. This setup offers the extra benefit of high speed measurements when compared with full spectral measurements.

The light source can again be chosen based on the type of object to be measured, but should always be appropriate for the colorimeter’s typical X, Y, Z characteristics. UV LEDs should not be used with colorimeters, but a white LED would be an ideal choice.

Figure 4. Overview of transmissive measurement procedure using a MSE colorimeter and Steropes light source. Image Credit: Admesy

## Transmissive Measurement Procedure Using Light Meter

If only luminance measurements are required, a light meter such as Admesy's Asteria and a reference light source would be most suitable. This setup allows precise measurements for filters with high density factors where measurements should be carried out at low luminance levels, or at very high speeds.

Figure 5. Overview of transmissive measurement procedure using an Asteria light meter and Steropes light source. Image Credit: Admesy

## General Recommendations for Transmissive Measurements

The following recommendations will help to ensure accuracy and repeatability of measurements:

• Control alignment of devices
• Control angle of measured object, typically at 90° on light beam direction
• Control distance between devices
• Control baseline with reference light source on a regular basis when performing several measurements
• Spectral properties of sample under test, detector and light source should be taken into consideration (e.g. measure NIR with halogen light source and detector that is equivalent the spectral area)
• Effects of luminescent objects should be considered
• Control stabilization of reference light source prior to setting baseline and measurements
• Control ambient lighting so that none enters the detector

This information has been sourced, reviewed and adapted from materials provided by Admesy.

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