Transmission spectra – making colors visible

Example spectra generated with Eureca-DIY spectrometer

A transmission spectrum describes which wavelengths of light can pass through a material and which are absorbed. When white light hits a sample, some of it is swallowed up (absorbed) and the rest is allowed to pass through (transmitted). We can analyze this transmitted light with a spectrometer and thus obtain the characteristic spectrum of the sample.

The measurement of transmission spectra is a central method in chemistry, physics, biology and materials science. Among other things, it enables

The identification of dyes and pigments
The analysis of liquids for their chemical composition
The examination of optical filters and glasses
Determining the purity of substances
Exciting experiments on light and color

How is transmission calculated?

To determine the transmission spectrum of a sample, we measure the transmitted light and set it in relation to a reference measurement. The transmission $T (λ)$ for a specific wavelength $λ$ is calculated as the quotient of the measurement signal $I (λ)$ and the reference signal $I_{0} (λ)$ :

T (λ) = \frac{I (λ)}{I_{0} (λ)} \cdot 100 %

$I (λ)$ : Intensity of the transmitted light through the sample
$I_{0}$ : Intensity of light without sample (reference measurement)
$T (λ)$ : Transmission in percent

The measuring process in detail

Reference measurement:: First, a reference measurement is carried out. The light is passed directly through a medium without a sample, e. g. through water (for liquids) or simply without a sample (for filters or glasses). The measured spectrum $I_{0}$ serves as a reference value.
Measurement of the sample:: The sample is then placed in the beam path. The spectrometer now measures the intensity $I (λ)$ of the light that remains after the interaction with the sample.
Calculation of the transmission:: The ratio of transmitted light is obtained by dividing $I (λ)$ by $I_{0} (λ)$ . Multiplying by 100 gives the transmission as a percentage.

Sample calculation

Assuming that the light of a certain wavelength has an intensity of $I_{0} = 5000$ in the reference measurement and an intensity of $I = 5000$ after passing through the sample, the following results:

T = \frac{25000}{50000} \cdot 100 % = 50 %

This means that the sample transmits 50 % of the light at this wavelength and the remaining 50 % is absorbed or scattered.

Interpreting a transmission spectrum

High transmission (e. g. 90 – 100 %)
→ The sample is almost transparent for this wavelength.
Medium transmission (e. g. 40 – 70 %)
→ The sample absorbs part of the light.
Low transmission (e. g. 0 – 20 %)
→ The sample absorbs or scatters almost all the light of this wavelength.

This method makes it possible to examine the spectral properties of dyes, filters or biological samples in detail!

Measurement of optical filters using transmission spectroscopy

Transmission spectroscopy is an essential method for characterizing optical filters. While volume filters have a broad but unspecific filter effect due to their absorption, interference filters enable a more precise selection of wavelengths. Both filter types have their specific advantages depending on the application and can be analyzed with our DIY spectrometer to determine their optical properties.

Simple experiments for school and university

There is a whole range of exciting experiments involving transmission measurements that are easy to carry out. Examples of these are

Floral dyes (anthocyanins):: How does the transmission spectrum change when the pH of the solution changes?
Tea and juice analysis:: What colorants are in fruit tea or red wine juice?
Sunglasses and UV filters:: How well do different glasses block UV light?
Examining food colors:: What colorants are in sweets?
Leaf dyes (chlorophyll & carotenoids):: Which absorption regions dominate the spectrum of a leaf extract?

These experiments not only help to understand the principle of transmission, but also make science a practical experience! We will gradually carry out these experiments with our DIY spectrometers and publish corresponding application descriptions here. If you are interested, follow us on LinkedIn or Instagram, for example!

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Last update: 2025-18-03