The edge of the filter is engraved with an arrow to indicate the recommended direction of the light transmission filter. Although any side of the filter facing the light source can serve as a light filter, it is better to face the coated side towards the light source. This can greatly reduce the thermal effect or possible thermal damage caused by the absorption of radiation outside the passband by the cutoff substrate or colored glass filter layer. A graph showing the functional relationship between the transmittance and wavelength measured by a filter illuminated by a low intensity broadband light source. It is shown that the direction of the transmission filter has little effect on the intensity and spectrum of the emitted light. The small difference in the incidence of light from the front and back is likely caused by small changes in the incidence angle when the filter is moved, flipped, and replaced.
When using a filter, the collimated light should be perpendicular to the surface of the filter. When uncollimated light or light is not incident perpendicular to the surface of the filter, the central wavelength (the wavelength corresponding to the transmittance peak) will move towards the short wavelength, and the shape of the transmission band (passband) will change. By slightly changing the incident angle, the passband of the filter can be effectively adjusted in a small range. When the incident angle changes significantly, it will cause a significant change in the central wavelength, and it will also significantly distort the shape of the passband, even leading to a significant decrease in the passband transmittance.
By changing the temperature of the filter, the central wavelength of the bandpass filter can be fine-tuned (~1 nm within the operating range of the filter). This is mainly caused by slight thermal expansion or contraction of the film layer.