Introduction to Visual Colors of Filters

In daily life, human vision can observe objects of various colors, and the colors of filters in optical components also include red, orange, yellow, green, blue, purple, and other colors. So, what determines the color of the filter lens?

Firstly, we need to understand the principle of light's action. The principle of light transmission on transparent and opaque media is different.

1、 The color of opaque media is determined by the colored light reflected by it. The color of opaque objects, such as tables and walls, is determined by the reflected color light. Opaque objects mainly reflect colored light that is the same as their own color.

For example, by shining white light on a blue object, the seven colors of red, orange, yellow, green, blue, indigo, and purple are absorbed by the blue object in the white light. Only blue light cannot be absorbed or reflected back, so we can see that the object is blue. But if red light is used to illuminate blue objects, because the red light is absorbed, we cannot see the reflected red light, resulting in blue objects appearing black visually. The color of other opaque objects is also the same.

In addition, white opaque objects can reflect all visible light, so no matter what color of light is emitted onto the white opaque object, we can see what color it is. Black transparent objects are difficult to reflect any color light, so any color of light on their surface will turn black because we cannot see the reflected color light.

For example, a red filter is an absorbing material. Under normal sunlight, it appears red, but when irradiated with a 532 green laser, it appears black.

2、 The color of an object in a transparent medium is determined by the color light it passes through

The color of transparent objects (such as glass) is determined by the color light they pass through, and transparent objects themselves can pass through the same color light.

For example, when white light passes through blue glass, except for blue light, other colors of light cannot pass through the glass smoothly, so the glass we see will display blue. If a transparent object allows all colored light to pass through, it will appear colorless.

For example, coated filters. It looks silver and white. If a red light is used, all red lights will pass, so it is red. If green lights are used, all green lights will pass through, so it looks green.

Ordinary coated filters are transparent, so their color depends on the wavelength band they choose to pass through. For example, a filter with a center wavelength of 400-450nm is purple, a filter with a center wavelength of 450-480nm is blue, and so on.