Transmission densitometer & OD and light transmittance measurement

1. Definition of optical density and light transmittance

Optical density: the light passing through the tested material, the energy difference is the energy absorbed by the tested material. At a specific wavelength, the concentration of the same material to be tested has a quantitative relationship with the energy absorbed.

Transmittance: Transmittance refers to the ability of light to pass through the medium. It is the percentage of the luminous flux transmitted through the transparent or translucent body and its incident luminous flux. When the false parallel monochromatic light passes through a uniform, non-scattering medium, part of the light is absorbed, partly passes through the medium and partly is reflected by the surface of the medium. Light transmittance can indicate the efficiency of the material to transmit light. In many industries, the light transmittance and optical density of materials have a prescribed range of qualification.

2. Conversion between optical density and light transmittance

Optical density abbreviated as OD. It is defined as the logarithm of the ratio of incident light to transmitted light or the logarithm of the light transmittance reciprocal. The calculation formula is OD = log10 (incident light / transmitted light) or OD = log10 (1 / transmittance). When testing materials in general, we find that the transmittance of some materials is very low. It is difficult to compare the differences between materials using the transmittance value. If the light transmittance value is converted into the optical density value, the data will be more intuitive.

3. Dual use optical density meter measure both OD and VLT

However, some customers think that after purchasing a light transmittance meter, it is necessary to convert the measured data to obtain the desired optical density value. In fact, we only need a transmission densitometer to measure two data at the same time to solve this problem. For some X films and IR inks, the measurement of light transmittance requires a professional transmission densitometer based on the diffuse transmission principle. Why do you say this way? This is related to the surface characteristics of the tested material. The editor has deliberately organized relevant knowledge on the measurement of light transmittance and optical density of diffuse transmission materials and hopes to provide help for everyone.

Transmission densitometer is an instrument that can measure optical density value and transmittance value at the same time. It uses the diffuse transmission principle and can replace the integrating sphere effect. This transmission densitometer is suitable for measuring translucent materials such as opalescent, frosted, etc. It can also measure high-transmittance materials, X film, aluminum coating and other materials.

The diffuse transmission principle: When light passes through a transmission material with a rough surface (such as frosted glass), the transmitted light diffuses. There is no regular transmission macroscopically, which is called diffuse transmission. If light shines on a light-transmitting material with good diffusivity (such as opalescent glass, etc.), the transmitted light will spread out in all directions and be evenly distributed in the entire hemispherical space. At this time, when the brightness is the same in all directions. This is called isotropic diffuse transmission (formerly known as uniform diffuse transmission)

When light is irradiated to translucent materials such as opalescent, frosted, etc., the light will diffuse diffusely instead of regularly transmitted on the tested material surface. This is determined by the surface characteristics of the object. If a general light transmittance meter is used to measure diffuse transmission materials, part of the light will be scattered. The instrument can’t receive all the light intensity. Therefore, for diffuse transmission materials such as opalescent, matte, film, etc., we should use the transmission densitometer using diffuse transmission principle.