Understanding the difference between Reflectance and Transmittance measurements

Users have requested clarity on the two Colour Measurement principles and how to know when to use each method:

• Reflectance Measurements
• Transmittance Measurements

Colour Measurement is a perception of the colour of an object when viewed under certain defined conditions.

The human perception is subjective, that is a human perception of colour will happen without “knowing the conditions” that apply at the time.
Two humans may observe the SAME object under DIFFERENT conditions, and therefore have different PERCEPTIONS of the colour and appearance of that object.

As white light (Sunlight) can be separated into discrete colours, each with its own wavelengths, colour perception can be reduced to a mathematical equation.
To ensure that the mathematical renderings are the same for all users and instruments, the formula is managed and controlled by the C.I.E. in Austria.

Reflectance and Transmittance are defined by the C.I.E. with precise definitions.

Reflectance Measurements:

REFLECTANCE: When light STRIKES an object, and when the object is opaque or not transparent, the light will be REFLECTED across the whole spectrum.
If the object appears white, 100% of the light has been reflected off the object.
If the object appears black, 0% of the light has been reflected off the object.

Colours in the spectrum will only be “seen” if the object REFLECTS a particular colour or Wavelength. We can see this in the diagram below:

Transmittance Measurements:

TRANSMITTANCE: When light PASSES THROUGH an object, and when the object is transparent or translucent, the light will TRANSMIT through the whole spectrum unchanged.
If the object appears red, then only the red spectrum has been “seen” while the rest of the spectrum passes through.

Both methods can measure colour, but it is important to understand how colour is measured using either reflectance or transmittance.

When to use Reflectance vs Transmittance?

Let’s take an object like Rooibos Tea as an example. You wish to determine the colour of the brewed tea as an indication of the strength of the tea leaves so you brew some tea and allow it to stand for a specified amount of time.

The liquid seems fairly opaque so you fill a glass tube cell with the tea and measure it in Reflectance on an instrument such as the Konica Minolta CM-5 Spectrophotometer.

Following the measurement, you look at the Spectral Reflectance Curve and the values of reflectance are low and the reflectance curve seems flat. In other words, most of the light has passed through the tea and very little has reflected onto the instrument’s detector.

Spectral Reflectance Curve scaled to a maximum reflectance of 2%. Essentially a flat line.

CIE L*a*b* data representing an almost ‘black’ colour. This indicates that most of the light was absorbed/transmitted through the sample.

You change the instrument settings to Transmittance and fill a glass transmittance cuvette with the tea. In this case, we have selected a cuvette with a 10mm optical path, although 2mm and 20mm optical paths can also be used.

In this case, a clear Transmittance curve is reported. This curve accurately which part of the visible spectrum has been detected and which wavelengths have not reflected.

Spectral Transmittance Curve showing an almost 90% light reflectance.

CIE L*a*b* data with an accurate representation of the colour of the tea. This indicates that transmittance is the correct measurement method for this sample.

Guidelines for understanding when to use Reflectance or Transmittance:

It is not always clear as to when you should perform reflectance measurement or when you should perform transmittance measurements. There are some guidelines users can follow when determining the optimal measurement condition.

Reflectance Measurements are typically used for the following objects:

• Solid objects – Paint contrast cards, ceramic tiles and opaque plastic and packaging
• Powders – Flour, spices, cement and pigments
• Opaque liquids – Sauces, condiments, yoghurts and cosmetic liquids
• Granules – Masterbatch pellets and malt or maize products

Transmittance Measurements are typically used for the following objects:

• Transparent or translucent – plastic films, transparent plastic or pre-forms
• Liquids – Juices, pulps, water, tea and coffee
• Gels – Cosmetic gels or bases

If you’re still unsure, we recommend starting with a reflectance measurement and should you receive a fairly flat reflectance curve, change to a transmittance measurement to find out which method provides the most accurate and consistent colour data.

We are here to help ensure that you receive the most reliable and accurate results from your colour measurement instrument so contact us for application support or scroll to The Narich Academy for quick convenient solutions to your colour measurement query or problem.