Factors affecting the transmittance of optical filters

The transmittance of the optical filter directly affects the performance of the filter in the optical system, and the factors affecting the transmittance are as follows:

1. The nature of the filter material itself
Absorption characteristics: Different materials absorb light to different degrees. For example, certain glass materials contain specific metal ions that absorb specific wavelengths of light. For example, glass containing iron ions will absorb ultraviolet light and some visible light, thereby affecting the transmittance of the filter in these wavelength ranges. Some organic materials also have their own unique absorption spectra, based on the filter made of organic materials, its transmittance will be limited by the absorption band of the material.
Scattering properties: Inhomogeneity within the material (such as tiny particles, density changes, etc.) can cause light to scatter. When the light propagates inside the filter material, the scattered light will deviate from the original direction of propagation and cannot pass through the filter smoothly, thereby reducing the transmission rate. For example, some optical materials containing impurities or bubbles will scatter light, especially in the case of short-wavelength light (such as blue light), the scattering phenomenon may be more obvious, because short-wavelength light is more easily scattered by small particles.

2. Coating of the filter
Coating material and number of layers: interference filter is through the interference of multi-layer film to achieve the filtering function. The refractive index and thickness of the coated material have a significant effect on the transmittance. For example, the use of alternating coatings of high-index and low-index materials can produce high-performance interference filters. The thickness of each film determines the conditions of interference, and the appropriate thickness can make the interference phase of a specific wavelength pass through. The number of coating layers is also critical, more coating layers can make the filter more selective to the wavelength, but at the same time may reduce the overall transmittance, because each layer of coating will have a certain light reflection and absorption loss.
Coating quality: The uniformity and density of the coating affect the transmission rate. If the coating is not uniform, thickness deviations may occur in some areas, causing the interference conditions in these areas to change, so that the transmission rate is different at different locations. An undense coating may contain tiny pores where light is scattered or absorbed, reducing the transmittance. For example, in the vacuum coating process, if the vacuum degree is not enough or the coating speed is not appropriate, it may lead to a decline in the coating quality.

3. Wavelength of light
Filters are usually wavelength selective, with different transmittance for different wavelengths of light. For example, bandpass filters only have a high transmittance in a specific wavelength range, and at wavelengths outside the passband, the transmittance is very low. Taking a green bandpass filter with a central wavelength of 550nm and a bandwidth of 40nm as an example, the transmittance is high in the 530-570 nm range, while outside this range, such as in the ultraviolet or infrared region, the transmittance is close to zero. This is determined by the design principle of the filter, whether it is based on interference or absorption filters, its filter mechanism for light is wavelength related.

4. The incidence Angle of light
When light hits the filter at different angles, the transmittance changes. In general, with the increase of the incidence Angle, the transmittance of the filter will decrease. For interference filters, the change of incidence Angle will lead to the change of light path in the film layer, thus affecting the interference conditions. For example, an interference filter with a transmittance of 90% at vertical incidence (incidence Angle 0°) may decrease to about 70% when the incidence Angle increases to 30°. Moreover, the change of the incident Angle may also cause the shift of the central wavelength, which in turn affects the transmittance at a specific wavelength.

5. Filter temperature

Temperature changes affect the physical properties of the filter material, such as refractive index and size. For interference filters, the change of refractive index will change the interference conditions of light in the film layer, thus affecting the transmittance. For example, some filters based on quartz materials, when the temperature rises, the refractive index of quartz will change, so that the originally designed transmission conditions for certain wavelengths of light are destroyed, and the transmittance is reduced. At the same time, temperature changes may cause the filter to expand or contract, which in the case of interference filters with precise coating thickness requirements, will change the thickness of the film layer, which in turn affects the transmittance.