定义：在很大波长区域的衰减因子近于常数的光衰减器。
中性密度滤光片是一种光衰减器，它在很大的波长范围内（例如，整个可见光区域或者一些红外区域）衰减程度近似相同。它与波长无关的性质将它去其它颜色滤光片区别开来，颜色滤光片在某一波长区域的衰减比其它波长时要大。
中性密度滤光片类似于灰色太阳镜，但是它们具有更高的光学质量以及更大的衰减程度。它们通常是矩形或者圆形，为了适应用户的需要。
衰减的强度通常由吸光率或光学密度来表征，是功率透射因子以10为底的对数的绝对值。例如，光学密度为3表示光功率的衰减因子为103 = 1000。
吸收和反射中性密度滤光片
一些中性密度滤光片是用玻璃制作的，玻璃中掺杂的材料会产生不需要的吸收过程。这些吸收滤光片有的没有涂层，有的具有宽带的抗反射涂层。
不是所有的滤光片都需要通过吸收产生衰减，还可以利用反射。常在玻璃上涂覆薄的金属涂层，可以同时提供吸收和反射。该滤光片的光学密度严格来说并不是吸光率，而是总的透射损耗。
这一金属滤光片的优势在于即使很薄的滤光片都可以得到很强的衰减。并且，可以得到很平坦的透射光谱。但是，存在的反射光可能在有些应用中会有影响，这时更适合采用非反射ND滤光片。
由于反射光不会使滤光片产热，原理上可以承担很高的光功率。但是，由于金属薄层同时会吸收大量的光，因此功率损耗阈值与吸收滤光片能承受的功率水平相当。
并且，加热时涂层会发生氧化反应。因此需要仔细的清洁过程，而纯玻璃滤光片则不需要。 在整个波长区域反射率可能不是平坦的，因为吸收是与波长有关的，但是透射率近于常数。
中性密度滤光片的应用
当环境光很强时，中性密度滤光片通常用在照相术中。该滤光片可以减小孔径大小，因此产生的像具有更深的焦点，或者滤光片可以延长快门开关时间这样可以得到由于移动产生的模糊效应。
照相术中更常用的是纯的吸收滤光片。还存在分度滤光片，其吸光率从顶部到底部是变化的。
在光学和激光器技术中，中性密度滤光片具有多种用途。例如，在很强的光进入光电探测器之前可以采用滤光片先进行衰减，或者还可以衰减一束激光光束。
但是，中性密度滤光片并不是高功率衰减器：应用到高功率中时会产生热效应使光束发生畸变，甚至会损坏滤光片。
将大的滤光片套件中的一些中性密度滤光片组成一条滤光片链，可以实现不同程度的衰减。将这些反射滤光器组合应用到激光光束中时，需要注意不能使光束垂直于滤光片。
原因在于很大的反射率可能会形成光学谐振腔。共振效应会极大的改变整体的透射率（与波长和间隔宽度密切相关），产生的高谐振腔功率会损坏滤光片。
经过矫正的具有很好的光学密度特性的中性密度滤光片，对于功率计非常重要，此时需要根据测量的透射功率计算入射的功率。
中性密度滤光片轮
可以采用可旋转的中性密度滤光片轮来实现连续调谐吸光率。这时，在绕着中心环绕一圈的过程中，吸光率会持续增加，直到某一点又重新回到其初始值。轮子通常用于光束面积只占据整个滤光器面积很小一部分的情形，这时光束的吸光率近于常数。
Acronym: ND filters
Definition: optical attenuators with an approximately constant attenuation in a substantial wavelength range
More general term: optical filters
Neutral density filters are optical attenuators which have an approximately constant degree of attenuation (filter loss) in a substantial wavelength range – e.g., throughout the visible spectrum or in some part of the infrared spectrum. This wavelength independence distinguishes them from color filters, which attenuate light in certain wavelength ranges substantially more than in others. They are similar to gray sun glasses, but in contrast to those they are usually flat, have a higher optical quality and often a higher attenuation. They are often provided in rectangular or circular shapes and may be fitted into holders for easier use.
The strength of attenuation is often quantified as an absorbance or optical density, which is the absolute value of the logarithm of base 10 of the power transmission factor. For example, an optical density of 3 means an attenuation of optical powers by the factor 103 = 1000.
Absorbing and Reflective Neutral Density Filters
Some neutral density filters are made from a glass which is doped with a material which provides the wanted absorption. Such absorbing filters may have either no coating or a broadband anti-reflection coating.
Filters do not necessarily attenuate by absorption; one can also utilize reflection. It is common to use a thin metallic coating on the glass, which provides both reflection and absorption. The optical density of such a filter is strictly speaking not an absorbance, but only quantifies the overall transmission loss. An advantage of such metallic filters is that strong attenuation is possible even with rather thin filters. Also, a very flat transmission spectrum is possible. On the other hand, the resulting reflections may be disturbing for some applications, where non-reflective ND filters are then more appropriate.
As reflected light does not heat the filter, it might in principle tolerate higher optical powers. However, as there is a substantial absorption in a very thin metal film, one may actually obtain surface damage at power levels which a purely absorbing filter would tolerate. Also, oxidation of the coating can occur when it is heated. Cleaning is also more delicate than with pure glass filters.
Note also that the reflectivity may not be flat at all in the wavelength region where the transmittance is nearly constant, because the amount of absorption is also wavelength-dependent.
Applications of Neutral Density Filters
Neutral density filters are often used in photography, when ambient light is too strong. Such a filter allows one to reduce the aperture size such that a larger depth of focus can be achieved for the images, or to increase the shutter opening time in order to obtain blur effects resulting from movements. Purely absorbing filters are most common in photography. There are also graduated filters where the absorbance varies e.g. from top to bottom.
In optics and laser technology, neutral density filters also find many applications. For example, one often uses them to attenuate intense light before it hits a photodetector, or to attenuate a laser beam. However, one should keep in mind that neutral density filters are in general no high-power attenuators: the application of high optical powers can lead to thermal beam distortions or even to laser-induced damage of the filter, at least locally.
By placing several neutral density filters from a large filter kit in series, one can realize many different degrees of total attenuation. When using such combinations of reflective filters for laser beams, however, one should be careful to avoid that the filters are hit in a perpendicular fashion. The reason is that the often substantial reflectivities lead to the formation of optical resonators. Resonance effects can strongly modify the overall transmission (of course, with a strong dependence on wavelength and gap width), and the resulting high resonant powers may destroy the filters.
Calibrated neutral density filters, with a well characterized optical density, can be useful e.g. for power meters, when the incident power must be calculated from the measured transmitted power.
Neutral Density Filter Wheels
A continuous adjustment of absorbance is possible with a rotatable neutral density filter wheel. Here, the absorbance may increase systematically along a circle around the center, until it jumps back to its original value. The wheel is often used for a beam the area of which covers only a tiny portion of the whole filter area, so that the absorbance within the beam is approximately constant.
