Biophotonics_Concepts_to_Applications

commonly is referred to as adichroic mirror,beam splitter,orbeam combiner.
Typically a dichroic mirror is placed at a 45° angle to the incoming light beam, as
shown in Fig.5.12.
Applications of dichroic mirrors are in areas such as confocal microscopes,
spectroscopy, laser beam delivery mechanisms,flow cytometry, and laser traps.
Figure5.13shows an example of the transmission and reflection performance for a
generic dichroic mirror, which is designed so that the spectral separation occurs at
905 nm. Below the design wavelength is a spectral band over which the device will
reflect more than 90 % of the incident light, that is, the average reflection
Rave> 90 %. Above the design wavelength is a spectral band for which the device
will transmit more than 90 % of the incident light, that is, the average transmission
Tave> 90 %. Such a design is called alongpass mirror.
The design is reversed forshortpass mirrors. In this case, the device will
transmit afinite spectral band below the design wavelength and will reflect another
longer-wavelength band above the design wavelength. Depending on the device
quality, actualfilters have varying degrees of cutoff sharpness between the two
spectral bands and exhibit some ripple ranging up to 10 % in the transmission or
reflection characteristics. The key dichroic mirror performance parameters are the
reflection band, the transmission band, and the 50 % design cutoff wavelength.
Table5.3lists typical values of key operating parameters for three commercial
longpass and shortpass dichroic mirrors.
Standard thinfilmfilters in the visible region are available as blue, green, yellow,
cyan, magenta, and red types. For example, a bluefilter passes 495±15 nm, a

Filter ripple < 10%

Filter ripple < 10%

50% longpass design

cutoff wavelength at

905nm

Reflection region

400-875nm 935-1300nmPass region

700 800 900 1000 1200 1300

20

40

60

80

100

Wavelength (nm)

Transmissivity (%)

Fig. 5.13 Example of a
longpass dichroic mirror
designed with a 905-nm
separation wavelength, which
has a 400–875-nm reflection
band and a 935–1300-nm
transmission band

Table 5.3 Transmission and reflection bands for three example commercial dichroic mirrors

Parameter 505 nm

Longpass

638 nm

Longpass

805 nm

Shortpass

Reflection band (Rave> 90 %) 380 – 490 nm 580 – 621 nm 820 – 1300 nm

Transmission band

(Tave>90%)

520 – 700 nm 655 – 700 nm 400 – 790 nm

50 % Cutoff wavelength 505 nm 638 nm 805 nm

140 5 Fundamentals of Optical Detectors