There are two types of ND (neutral density) filters: absorptive and reflective. The absorptive type absorb light that is not transmitted, while the reflective type reflect it away. An absorptive ND filter has greatly reduced back reflections when compared to its reflective counterpart. This can be very important for various applications that are severely affected by back reflections, such as electronic imaging. However, since absorptive filters tend to absorb the light passing through them, the result is a slight increase in temperature. If critical temperature control is a factor in your application, we recommend using a reflective ND filter. Also, in absorptive ND filters, optical density is a function of glass thickness - the higher the optical density required, the thicker the filter will need to be. Reflective ND filters gain their properties through a coating material, allowing for thinner substrates and tighter thickness tolerances regardless of optical density. Care should be taken when using the reflective type of ND filter to insure that reflected light does not interfere with the application. In stacks, the reflective filters are not parallel in order to reduce back reflections.