The Difference Between UV Transmission and UV Absorbance

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The Difference Between UV Transmission and UV Absorbance

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Many UV instruments, like the DR 6000 Spectrophotometer and online UVAS plus sc probe from Hach Lange, offer two methods using the UV/254 nm wavelength for measuring organic constituents in a water sample: UV transmission (UVT) and UV absorbance (A254). At first glance, the methods appear to be complementary to each other.

This is true as the UV/254 Transmission Method measures the light transmitted or passed through a sample while the UV/254 Absorbance Method (A254) measures the amount of UV light that is absorbed by the sample.

However, while both methods are often referenced as measuring organic constituents, the models for each method have different applications and it is important to understand the meaning and use of the value obtained.

When light is passed through a sample, a portion of the light is absorbed or scattered by substances present in the sample. The amount of light energy exiting the sample is less than the initial incident light applied.

The light reaching the detector is the transmitted light (IT) or transmittance. The ratio of the transmitted light (IT) to the incident light (Io) is the transmittance (T) expressed as %T.

UVT is assumed to mean the %T at 254 nm unless noted otherwise.

The incident light that is absorbed or scattered by the sample is called the absorbance (A). The absorbance and transmittance values are related by the logarithmic equation of A = log10 (1/T).

To determine which method to use, one needs to ask if you need to measure the amount of light reaching the detector (typical in a UV disinfection application) or if you are interested in measuring the properties of the sample that are absorbing the 254 nm light (typical in controlling disinfection byproducts).

UV Transmission Method – UV Disinfection Applications
The UV transmission method is used where there is a need to measure the amount of light reaching a detector or sensor. 

For example, this method would be used to determine the UV lamp output settings on continuous flow UV reactors. The UVT value is also used for the compliance monitoring of UVT analyzer calibrations and in the determination of UVT water quality characteristics for designing UV treatment facilities. These applications require UVT values of the water to insure the UV light dose delivery is sufficient to inactivate pathogenic microorganisms and to insure that UVT analyzers are properly calibrated to insure the analyzers accurately measure and control the intensity of the UV/254 light being delivered.

It is important to note that in these types of unfiltered samples, the UVT value is determined by the sample color, turbidity, particles and the soluble organic or inorganic compounds present in the water. A full list of interferences and additional precautions is given in the “Organic Constituents UV Absorbing (UV-254), Method 10054” DR 6000 procedure manual (more information at

UV Absorbance Method – Disinfection Byproduct Control
The UV Absorbance Method (A254) is commonly used to characterize the organic constituents in a water sample.

Research has shown that the organic compounds absorbing at 254 nm have a greater tendency to form chlorinated disinfection byproducts when contacted with chlorine in the disinfection of drinking water.

Source waters can be monitored with this method to predict the DBP formation potential or can signal major changes in source water quality. With this method the turbidity and particles in the water that absorb or scatter the 254 nm light are removed by first filtering the water through a 0.45 micron filter before making the absorbance measurement.

It is important to note that some methods call for adjusting the sample pH before measuring the A254 absorbance. However, when the A254 value is used for other UV disinfection applications, the water sample should be analyzed without filtering or adjusting the pH.

The UV/254 absorbance value is also used in the calculation of Specific Ultraviolet Absorbance (SUVA). SUVA is defined as the ratio of the A254/cm to the total DOC concentration in the filtered sample. The DOC is determined by running a TOC test on the filtered sample.

SUVA  (L/mg-M) =  A254/cm  /  DOC (mg/L) x 100 cm/M

The SUVA value is a measure of the dissolved organic carbon concentration available to likely form disinfection byproducts when chlorinated. The value is also used to removal of natural organic matter in the reduction of disinfection byproducts.

Normalization calculation
A254 and UVT measurements are normalized to one centimeter and expressed as Abs(254)/cm or % UVT/cm respectively. The cm designation refers to the length of the quartz sample cell pathlength used to make the measurement.

Measurements made on higher quality waters often have a low absorbance or high transmittance levels. Longer pathlength 5 cm or 10 cm cells are often used to obtain better measurement sensitivity and accuracy.

The HACH LANGE DR 6000 method programs automatically normalize the linear absorbance and logarithmic transmittance values to equivalent Abs/cm or percent UVT/cm values respectively, when the longer pathlength cells are used avoiding the need to manually do the normalization calculations.

The UV Transmittance and UV Absorbance methods both offer the ability to help understand your water and effectively manage your treatment processes. However, it is important to understand the meaning and use of the value obtained. 

If you have any further questions about these measurements, please contact your local sales representative or visit


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