Comparing Chroma’s AT and ET Filter Sets
Often, Chroma Technology’s AT filter sets are a lower cost option than their ET filter sets.
AT filter sets are the perfect choice for routine fluorescence detection and assays, as well as for most applications requiring accurate color discrimination and excellent out-of-band blocking. ET filter sets are Chroma’s highest quality option. They have zero-pixel-shift image registration, and are necessary for applications requiring the highest possible signal intensities.
Using an emission filter as an excitation filter
Chroma’s excitation filters are made to block all light from 300–1000 nm or higher outside of the bandpass. Most of their emission filters do not block light that far out of the transmission band and therefore might pass some unwanted light from your light source to your sample. More importantly, this unblocked excitation light will then be transmitted to the detector.
If using an ET filter, it is very important to ensure that the filter is adequately blocking all of the excitation light transmitted to the sample. When filters are requested that are very close in wavelength, Chroma engineers look not only at transmission, but at the optical density (blocking) measurements to be sure there will be no unwanted excitation light being transmitted to your detector.
When blocking (that is attenuation), the emission filter does not sufficiently block all wavelengths of the excitation light, then this light will be transmitted to the detector. Keep in mind that the intensity of the excitation light is many orders-of-magnitude greater than that of the fluorescence emission from a sample labeled with fluorophores. This can appear as anything from a light colored haze to an almost blindingly bright sea of light, obscuring any fluorescence. Furthermore, unblocked excitation light may lead to destruction of equipment or damage to the user’s eyesight.
Do I need an excitation filter with my single-color LED?
Yes, an excitation filter is required. The output of LED light sources is not monochromatic and instead has a Gaussian shaped spectral emission profile. The “tails” of this LED emission often extend >40 nm from the peak wavelength and must be excluded from the passband of the emission filter. An excitation filter will block these unwanted wavelengths and transmit only the desired wavelengths to your sample.
Do I need an excitation filter with my laser?
Yes, Chroma recommends an excitation (laser clean up) filter. Few lasers are truly monochromatic. Gas lasers typically emit several discrete wavelengths and the blocking provided by Acousto-Optic Tunable Filters (AOTFs) is often insufficient to prevent laser noise in the emission channel. Many laser diodes can have significant bandwidth and should be treated as a broadband source. Also, because laser illumination is very intense, relatively low levels of laser light can dramatically affect the signal/noise ratio of a fluorescence image.
Using a longpass emission filter as a dichroic
All of Chroma’s filter and dichroic designs are very angle sensitive. Their filters are designed for a zero degree angle of incidence (AOI) and dichroics are designed for a 45 degree AOI (unless otherwise specified). Changing this angle will drastically shift the spectral performance of the component.
Filters for flow cytometry or plate reader applications
Because of the multiplexing typically employed in flow cytometry, you’ll want to use filters which are fully blocked out-of-band up to about 900 nm, which is the detection limit of most PhotoMultiplier Tubes (PMTs). Typically, almost all of their excitation filters and many of their emission filters will work well in these applications. Chroma simply needs to know the wavelength range that you want the filters to transmit, the wavelength range of the excitation light (typically laser or LED), and the size of the desired filter.
About Chroma Technology
Chroma Technology is a leading manufacturer and OEM supplier of high performance optical filters used in a wide range of applications, including health care, biomedical research, remote sensing, machine vision, and aerospace. Chroma offers an extensive selection of optical filter sets and filter accessories, as well as custom solutions tailored to customer-specific needs.
For more information or a quotation for this or other Chroma products, contact IL Photonics.