What is Stray light and how it is monitored?

What is Stray light and how it is monitored?

Despite being a vital specification, stray light is mostly overlooked in a correctly calibrated spectrophotometer. It happens because users don’t know what exactly it is and how it gets monitored. If you fall under this crowd, it is time that you learn more about the stay light. 

In this article, we have included that you need to know about the stray light. So you won’t ignore it the next time while using the spectrophotometer.

What Is Stray Light?

A Spectrophotometric detector responds to the total light energy reaching it without discriminating between the individual wavelengths. Stray light is any light reaching the detector that lies outside the wavelength band width selected for analysis by the monochromator. It arises due to light scatter, diffraction by optical components or even by sample or on account of instrument malfunction.

In simpler words, we can also state that stray light is electromagnetic radiation that isn’t necessary for the spectrophotometer analysis and only interferes with the process. A few examples of it can be:

  • The light coming from any gaps in the spectrophotometer’s fold. 
  • The light reflections coming from mechanical mounting surfaces present inside the system. 
  • Any imperfections on the system’s surface, such as dust, scattering unnecessary light, etc.
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Stray light standards (Image Courtesy : https://www.whitebearphotonics.com/)

Stray light introduces an error in the measured absorption signal. The absorbance readings begin to drop due to increase in stray light and leads to a negative deviation from the Beer- Lambert’s law which is the basis for quantitative estimations in UV – VIS spectroscopy.

The effect becomes significant at higher concentrations as the stray light component becomes a larger part of the total transmitted light at such concentrations. In other words stray light reduces the linearity of response of the instrument.

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Stray light can pose problem at any wavelength but an increase in stray light becomes significant in the UV range where the energy throughput of the instrument is relatively low.

Types of Stray Light

The stray light spectrophotometer is typically divided into two types: Ghosts and Flare. Let’s understand them in depth. 

Ghost Stray Light: Have you ever noticed an unwanted glare in your images due to multiple reflections from a light source? This is called the ghost light. It mostly happens when reflections occur between imaging surfaces. We can also call it a second image. 

Flare Stray Light: Unlike the reflection in ghost light, here stray light is created due to scattering. This scattering of light can happen because of the presence of certain imperfections in the system, such as mechanical elements. These are also called the veiling glare.  

Do All Spectrophotometers Have Stray Light?

Most people have a concern about whether stray light spectrophotometer is common for all equipment or their tool has some problem. Well, to their relief, all spectrophotometers have some limit of stray light.

It is usually caused due to diffraction, scattering, or a problem in the machine. If you want to minimise the stray light in your spectrophotometer, you will have to opt for the higher-end models. They are designed in a way so that they provide more accurate results with minimal interference. 

Still, you need to remember that there will be some limit of stray light present in your tool, irrespective of what model you choose. Moreover, this issue won’t go away with a change in the spectral range. You will find stray light in UV, VIS, and NIR. The stray light has to be checked every now and then because it gets worse with time. 

Now you can move on to monitoring stray light in UV, NIR, or VIS spectrophotometer ranges. 

Stray Light Monitoring

Stray light is measured by making use of cut-off filters. Such filters absorb light at the wavelength at which measurement is to be performed but transmit higher wavelengths. Thus any light transmission below the cut-off wavelength is due to stray light

ASTM Procedure

This procedure is adopted for measurement of stray light transmittance at three different wavelengths, namely, 220nm, 340 nm and 370 nm so as to observe stray light over a wider range of wavelengths. Cut-off standard sealed cuvettes containing following solutions are used:

10g/L sodium iodide for 220 nm

50 g/L sodium nitrite for 340 nm and 370 nm

These solutions have a sharp cut-off in the UV region so if any light is detected below these cut-off values it is stray light.

These can also be found in the market as liquid stray light calibration filters. These have been preferred by experts for years. The only way they fall short is due to their limited wavelength filtration. The stray light must go by that wavelength, or it won’t get tested out. 

However, now there are also solid stray light calibration filters available. The significant benefit of these filters is that they can test the stray light at almost all possible wavelengths. Therefore, these options are more efficient for the purpose. 

Pharmacopoeial Procedure

The European Pharmacopoeia recommends measurement of absorbance of a standard solution of 12 g/L solution of Potassium chloride at 198 nm and the absorbance reading should be 2A or higher.

It can be is seen that stray light is an important instrument specification for its ability to correctly reflect accuracy of spectrophotometric measurements at higher absorbance values.

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