Transmission Spectroscopy

Abstract: in this post we describe the application of the Thunder Optics SMA Spectrometer and the Spectragryph software in transmittance measurements. We will apply the technique of transmission spectroscopy to the study of some optical filters.

Introduction

In the previous post: Thunder Optics Spectrometer & Spectragryph, we described the Thunder Optics SMA Spectrometer (hereafter referred to as the TO spectrometer) and used it to acquire the spectra of some light sources. We now continue the experimentation activity by “exploring” with this equipment, with its accessories and with the spectragryph software, the technique of transmission spectroscopy. In particular we will use this technique to analyze the optical characteristics of a set of optical filters.

The Measurement of Transmittance 

The transmittance (generally indicated with τ or T), in optics and in spectroscopy, represents the  effectiveness of a material in transmitting the incident light. Therefore, being the ratio between the intensity of the transmitted light and the intensity of the incident light, it is a dimensionless quantity. We can calculate this quantity with the following relationship:

T = Φ_t / Φ₀

Where Φ₀ and Φ_t are the incident light and the emerging light from the sample under examination. Transmittance is often expressed as a percentage value: T% = 100T.

Spectragryph software automates the calculation of transmittance. The selected measurement mode defines the type of y-axis of the measured live spectrum: intensity, transmittance, reflectance, absorbance (Fig. 1). We will choose the transmittance mode.

Fig. 1 – Measurement Mode Selection

Depending on the chosen modality, one or more auxiliary spectra may be required. The auxiliary spectra are the Dark spectrum, the Reference spectrum and the Blank spectrum. Each of these can be set, removed and viewed at any time. As soon as they are registered, they are stored and kept ready for later use. To update them, simply set them up again with a newly measured live spectrum. The Dark spectrum (Fig. 2) and Blank spectrum (Fig. 4) are optional, so their use must be activated by clicking on the respective button. The Reference spectrum (Fig. 3) is always mandatory except for the intensity mode, when necessary it is used automatically by the system.

Fig. 2 – Dark spectrum assignment

Fig. 3 – Reference spectrum assignment

Fig. 4 – Blank spectrum assignment

Measurement mode of auxiliary spectra:

• Dark spectrum: light source off, shutter closed, no light reaches the detector, mode: intensity
• Reference spectrum: light source on, full light (100% level) reaching the detector, mode: intensity
• Blank spectrum: with “blank” sample (eg pure solvent or buffer in the sample container), with the final measurement mode selected

Each spectrum should be re-measured after changing the exposure time, furthermore the reference spectrum should be updated after any change in intensity of the excitation light.

The reference light source should have a spectrum as flat as possible over the entire range of wavelengths of interest. Lamps of this type are for example halogen or xenon lamps. If such a lamp is not available, a different light source can also be used, as long as it has a constant intensity over time. We used Thunder Optics Mini Light Source.

Fig. 5 – (a) Dark spectrum (orange line) and live spectrum (blue line) (b) Reference spectrum (orange line) and live spectrum (blue line)

After the definition of the reference spectrum, the measurement mode of the spectrometer can be switched to transmittance, where the transmittance calculated from the raw measurement and the reference spectrum is shown directly on the graph (Fig. 6). To improve the accuracy of the measurement it is always recommended to acquire the dark spectrum and activate its use, the same applies to the blank spectrum. Generally for each measurement context it is necessary to evaluate which auxiliary spectra has to be acquired and used. In general, the formula used by the software for calculating the transmittance is the following:

Transmittance: Live = (Raw – Dark) / (Reference – Dark) – Blank

Fig. 6 – Transmittance spectrum recorded without sample

Materials and Methods

For the study of transmittance (for example of optical components such as filters) we can use the TO spectrometer both in direct coupling with a light source, as shown in Fig. 7, and using the optical fiber and the cuvette holder (Fig. 9) . In the first case the filter must be placed between the SMA connector of the spectrometer and the SMA connector of the light source, in the second case the filter can be positioned in the cuvette holder and held in place by screwing the collimator (Fig. 8).

Fig. 7 – (a) Spectrometer and light source directly coupled (b) Detail of the optical filter

Fig. 8 – (a) Cuvette Holder (b) Cuvette holder with optical filter in the receptacle

Fig. 9 – (a) Cuvette holder (b) Light source seen through an optical filter placed inside the Cuvette holder

Optical Filters

An optical filter is a device that selectively transmits light of different wavelengths. They are usually made with a glass or plastic plane inserted into the optical path. The optical properties of filters are described by their “frequency response”, which specifies how the intensity of each wavelength of incoming light is attenuated by the filter.
There are two main categories of filters: those based on the absorption of light by the material that constitutes the filter and those based on interference or dichroic filters. The filters most used in optical spectroscopy applications are the interference filters; these filters are made by depositing a series of optical coatings on a glass substrate. These layers form a sequential series of reflective cavities that resonate at the desired wavelengths, while the other wavelengths cancel out destructively or reflect when the peaks and minima of the waves overlap (Fig. 10-a) .

Fig. 10 – (a) Scheme of interferential filter (b) Filter analysed in this paper

Two classic uses of optical filters are those in fluorescence analysis and Raman spectroscopy. Fluorescent substances have an absorption spectrum and an emission spectrum: in order to keep the excitation light and the emission light separate, two bandpass filters are used as shown in Fig. 11. In Raman spectroscopy there is the same need to collect only the diffused light due to the Raman effect, excluding the excitation light source with suitable optical filters.Fig. 11 – Excitation and emission filters for the study of fluorescence

Bandpass Optical Filters Analysis

Bandpass (BP) filters are optical filters that allow one or more specific wavelength bands to pass while blocking others. Bandpass filters are characterized by the range of wavelengths they transmit, also known as the pass band. They are quite common filters widely used in many optical applications, among which we mention: environmental analysis, colorimetry, flame photometry, fluorescence applications, UV sterilization, spectral radiometry, medical diagnostics, chemical analysis, computer vision, biotechnological instrumentation and medical devices.
With our spectrometer and our experimental setup we have analyzed a series of optical filters of the bandpass type, obtaining the transmittance spectra presented in the following diagrams.

Fig. 12 – Bandpass filter transmittance: band 430 – 480 nm

Fig. 13 – 530 nm bandpass filter transmittance

Fig. 14 – 630 nm bandpass filter transmittance

Fig. 15 – 660 nm bandpass filter transmittance

Longpass Optical Filters Analysis

A longpass (LP) filter is an optical interference or colored glass filter that attenuates the shortest wavelengths and transmits the longest wavelengths within the range of the target spectrum (ultraviolet, visible or infrared). Longpass filters, which can have a very sharp step (edge ​​filters), are characterized by the wavelength at which the intensity is 50% with respect to the peak value. Longpass filters are frequently used in spectroscopy and fluorescence microscopy, as dichroic mirrors and as barrier filters (to select the emission light). In the following diagrams we present the transmittance spectra of a series of longpass optical filters obtained with our TO spectrometer and experimental setup.

Fig. 16 – 480 nm longpass filter transmittance

Fig. 17 – 560 nm longpass filter transmittance

Fig. 18 – 700 nm longpass filter transmittance

Notch Optical Filters Analysis

Notch filters are optical filters that selectively block a portion of the spectrum, transmitting all other wavelengths. Notch filters are used for example in Raman spectroscopy, to eliminate the excitation wavelength. In the following diagram we show the transmittance spectrum of a notch filter that blocks wavelengths around the value of 532 nm.

Fig. 19 – 532 nm notch filter transmittance

Conclusions

Our apparatus consisting of Thunder Optics SMA spectrometer and mini light source proved to be more than adequate for the qualitative and quantitative analysis of the transmittance of the optical filters we examined. Analyzes of this type generally make use of halogen or xenon light sources, which guarantee an intense emission with a “flat” spectrum compared to an incandescent lamp. The spectrometer and the spectragryph software have however adequately compensated the non-constant spectrum of the light source used, allowing to obtain excellent results.

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