Neutral Density Filter

definition:
An optical attenuator with an attenuation factor close to a constant in a large wavelength range.

A neutral density filter is a type of optical attenuator that attenuates approximately equally over a wide wavelength range, such as the entire visible light region or some infrared regions. Its wavelength independent property distinguishes it from other color filters, where the attenuation of a color filter is greater in a certain wavelength region than in other wavelengths.
Neutral density filters are similar to gray sunglasses, but they have higher optical quality and greater attenuation. They are usually rectangular or circular in shape to meet the needs of users.  
The intensity of attenuation is usually characterized by absorbance or optical density, which is the absolute value of the logarithm of the power transmission factor with a base of 10. For example, an optical density of 3 represents an attenuation factor of 103=1000 for optical power.  

Absorption and reflection neutral density filter
Some neutral density filters are made of glass, and the materials doped in the glass can cause unwanted absorption processes. Some of these absorption filters are uncoated, while others have broadband anti reflective coatings.  
Not all filters require attenuation through absorption, reflection can also be utilized. Thin metal coatings are often applied to glass to provide both absorption and reflection. Strictly speaking, the optical density of this filter is not the absorbance, but the total transmission loss.
The advantage of this metal filter is that even very thin filters can achieve strong attenuation. Moreover, a very flat transmission spectrum can be obtained. However, the presence of reflected light may have an impact in some applications, making it more suitable to use non reflective ND filters.  
Due to the fact that reflected light does not cause the filter to generate heat, it can theoretically withstand high optical power. However, due to the metal thin layer absorbing a large amount of light at the same time, the power loss threshold is comparable to the power level that the absorption filter can withstand.
And during heating, the coating will undergo oxidation reaction. Therefore, a careful cleaning process is required, while pure glass filters do not.  
The reflectivity may not be flat across the entire wavelength range, as absorption is wavelength dependent, but the transmittance is close to a constant.  

Application of neutral density filter
When the ambient light is strong, neutral density filters are usually used in photography. This filter can reduce the aperture size, resulting in a deeper focus of the image, or the filter can extend the shutter opening and closing time to obtain the blurring effect caused by movement.
Pure absorption filters are more commonly used in photography. There are also graduated filters, whose absorbance varies from top to bottom.  
Neutral density filters have multiple applications in optics and laser technology. For example, a filter can be used to attenuate strong light before it enters the photodetector, or a laser beam can also be attenuated.
However, neutral density filters are not high-power attenuators: when applied to high power, they can generate thermal effects that distort the beam and even damage the filter.  
Combining some neutral density filters from a large filter kit into a filter chain can achieve varying degrees of attenuation. When combining these reflective filters into a laser beam, it is important to be careful not to make the beam perpendicular to the filter.
The reason is that a high reflectivity may form an optical resonant cavity. The resonance effect will greatly change the overall transmittance (closely related to wavelength and spacing width), and the high resonant cavity power generated will damage the filter.  
A neutral density filter with good optical density characteristics after correction is very important for power meters, and the incident power needs to be calculated based on the measured transmission power.  

Neutral density filter wheel
A rotatable neutral density filter wheel can be used to achieve continuous tuning of absorbance. At this point, during the process of circling around the center, the absorbance will continue to increase until it returns to its initial value at a certain point. Wheels are usually used in situations where the beam area occupies only a small part of the entire filter area, and the absorption rate of the beam is close to a constant.