Absorption Type Neutral Density Filter
Absorption Type Neutral Density Filter
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What are neutral density filters
- A neutral density filter is a filter that reduces the intensity of light equally across all wavelengths, without changing the color or hue of the light.
- Neutral density filters are used to control the exposure, depth of field, and motion blur of an image or a laser beam.
- Neutral density filters can be either colorless (clear) or grey, and they can have different ratings or densities that indicate how much light they block.
- Neutral density filters can also have different shapes and sizes, such as circular, square, rectangular, or custom-made.
Major Specifications of Neutral Density Filter
- Density: The density of a neutral density filter is a measure of how much light it reduces. It is usually expressed as a fraction (e.g., 1/2, 1/4, 1/8) or a decimal number (e.g., 0.5, 0.25, 0.125). The higher the density, the darker the filter and the less light it transmits.
- Optical Density (OD): The optical density of a neutral density filter is another measure of how much light it reduces. It is related to the density by the formula: OD = log10(1/density). For example, a filter with a density of 1/10 has an OD of 1, and a filter with a density of 1/100 has an OD of 2. The higher the OD, the darker the filter and the less light it transmits.
- Transmission: The transmission of a neutral density filter is the percentage of light that passes through it. It is related to the OD by the formula: transmission = 10^(-OD). For example, a filter with an OD of 1 has a transmission of 10%, and a filter with an OD of 2 has a transmission of 1%. The lower the transmission, the darker the filter and the less light it transmits.
- Filter Factor: The filter factor of a neutral density filter is the ratio of the exposure time required with and without the filter. It is related to the transmission by the formula: filter factor = 1/transmission. For example, a filter with a transmission of 10% has a filter factor of 10, and a filter with a transmission of 1% has a filter factor of 100. The higher the filter factor, the darker the filter and the more exposure time it requires.
- Size: The size of a neutral density filter is the diameter or dimensions of the filter. It depends on the type and model of the lens or optical device that it is attached to or inserted into. The size should match or exceed the size of the lens or optical device to avoid vignetting or cropping.
- Shape: The shape of a neutral density filter is either circular or rectangular/square. Circular filters are usually screwed onto the front of the lens, while rectangular/square filters are usually slid into a holder that is attached to the lens. Circular filters are more convenient and compact, while rectangular/square filters are more versatile and adaptable.
Reflective VS Absorption Type of Neutral Density Filter
The difference between reflective type and absorption type of neutral density filter is how they reduce the intensity of light. Reflective type filters use partial reflection to block some light, while absorption type filters use partial absorption to block some light.
Reflective type filters have a thin film coating, usually metallic, that reflects some light back to the source. They can be optimized for specific wavelength ranges, such as UV-VIS or NIR. However, they may cause unwanted back-reflections or interference in the system.
Absorption type filters have a glass substrate that absorbs some light within the material. They follow the Beer and Bouguer's Laws, which relate the absorption to the thickness and concentration of the filter. However, they may generate heat or change color due to the absorption.