Understanding ND Filters
Neutral Density (ND) filters are designed to evenly reduce light transmission across a specific part of the spectrum. These filters are typically defined by their Optical Density (OD), which quantifies the amount of energy blocked.
A higher optical density value corresponds to lower transmission, while a lower optical density indicates higher transmission (Equations 1 – 2).
\[\tag{1}T(\text{Percent Transmission}=10^{-OD}\times100\%\]
\[\tag{2}OD=-\log\left(\frac{T}{100\%}\right)\]
Example 1: What is the transmission if OD 0.3 and OD 1.5 filters are stacked?
\[\tag{3}OD_\text{total}=0.3+1.5=1.8\]
\[\tag{4}T=10^{-1.8}\times100\%=1.58\%\]
Common uses for ND filters include evenly reducing light across the visible or near-infrared spectrum, protecting sensors from overly bright light that can lead to blooming or overexposure in CCD cameras, and evaluating the linear response of photodetectors and photodiodes.
In situations involving high light intensity, ND filters are often preferred over polarizers to effectively reduce brightness without impacting color fidelity.
Types of Neutral Density Filters
ND filters come in two primary types: reflective and absorptive.
Reflective ND filters are constructed with thin film optical coatings, usually metallic, applied to a glass substrate. These coatings can be fine-tuned for specific wavelength ranges such as UV-VIS or NIR, and they mainly reflect light back toward the source. Care must be taken to prevent the reflected light from interfering with the system setup.
Absorptive ND filters work by absorbing light that isn’t transmitted, whereas reflective ND filters redirect the light away. One advantage of absorptive ND filters is their significantly reduced back reflections compared to reflective filters, which is crucial for applications like electronic imaging that are sensitive to such interference.
However, absorptive filters absorb the light passing through them, causing a slight increase in temperature. For applications where temperature control is critical, using reflective ND filters is recommended.
In absorptive ND filters, the optical density is determined by the thickness of the glass—higher optical densities require thicker filters. In contrast, reflective ND filters achieve their properties through a coating material, which allows for thinner substrates and more precise thickness tolerances, regardless of the optical density.
When using reflective ND filters, it is important to ensure that reflected light does not interfere with the application. To minimize back reflections when stacking reflective filters, they are arranged non-parallel.
Neutral density (ND) filters combine their optical density values additively. For example, stacking filters with optical densities of 0.6 and 0.9 results in a total optical density of 1.5, corresponding to an overall light transmission of 3%.
Example 2: How can I build a filter with 0.5% Transmission?
\[\tag{5}OD=-\log\left(\frac{0.5\%}{100\%}\right)=-\log(0.005)=2.3\]
ODTotal of 2.3 could be created by stacking OD 0.3 + OD 2.0 or OD 1.0 + OD 1.3.
How to orient reflective ND or interference filter?
When positioning reflective ND (neutral density) or interference filters, they can function correctly regardless of which side faces the light source. However, for optimal performance, we recommend orienting the side with the “mirror-like” reflective coating toward the light source. This orientation helps to minimize thermal effects, as it prevents the glass on the opposite side from absorbing heat.
Additionally, if the “mirror-like” side faces away from the source, it can create interference patterns, especially when exposed to a coherent light beam. To avoid such issues, always position the coated surface facing the source. This principle generally applies to filters of all types to ensure they perform as intended.
For the best results, filters should be placed in a collimated beam of light. This orientation reduces the angle of incidence, allowing the filter’s performance to more closely align with its design specifications. Interference filters, in particular, are highly sensitive to the angle of incidence, so careful positioning is essential for maintaining their optimal performance.
Continuously Variable Apodizing Filter
A Continuously Variable Apodizing Filter is a specialized type of neutral density filter that features a gradient of optical densities varying radially from the center. This filter is available in two main configurations: one with a high optical density at the center that gradually decreases towards an uncoated edge, and another with high optical density at the edge that progressively decreases towards an uncoated center.
