Understanding Shortpass Filters

Edge filters, including both shortpass and longpass filters, are used to transmit one range of light while blocking or rejecting another. For a shortpass filter the transition point between transmission and rejection is known as the cut-off wavelength: the wavelength at which transmission is at 50%. The cut-off wavelength is the most important specification of a shortpass filter and is given by a simple number, for instance, 600 nm.

A well designed shortpass filter will have a steep cut-off, and above the cut-off wavelength light will be effectively blocked. Almost 100% of light shorter than the cut-off wavelength will be transmitted without change. These ranges are referred to as the rejection and transmission ranges of the shortpass filter.

Mounting Your Shortpass Filter

While shortpass filters are able to filter light in either direction, with negligible difference in intensity or spectrum of light transmitted, we recommend that you place your filter with the coated side facing the light source. Not only will this eliminate the risk of thermal damage to your filter, it will also decrease any thermal effects that might be caused by intense out-of-band-radiation.

Our standard shortpass filters are commonly used with a normal angle of incidence, although orientation can be changed to fine-tune the cut-off wavelength. As a rule of thumb, for every 45 degree of incidence change the cut-off wavelength will change by 10%.

Applications of Shortpass Filters

Shortpass edge filters play an important part in many imaging and photometry devices, and can be very helpful in eliminating distortion by blocking large ranges of unwanted light;. For instance, one common use is as a hot mirror, blocking undesirable infrared radiation. These filters are important in Raman spectroscopy and fluorescence microscopy. Shortpass filters can also be used in combination with longpass filters to form bandpass filters. The filters are arranged so that radiation passed through one filter enters the other, and thus both high and low wavelength ranges are eliminated and the filter combination will transmit only the desired passband.

At Shanghai Optics we carry a wide range of standard and hard coated shortpass filters in stock, individually and in kits. Our experienced engineering and design team is also available to help you design the ideal filter arrangement for your application. Please contact us to receive a listing of in-stock shortpass filters and to discuss custom options.