15
instruments can be eliminated by a monitoring system which measures the intensity and adjusts
the signal amplifier gains so as to be inversely proportional to the intensity (Steen 1980).
Under field conditions, arc lamps have an advantage in their low power consumption and
resistance to mechanical shock and vibration.
Clearly, both lasers and arc lamps have their advantages and disadvantages. Lasers can
produce higher intensities than arc lamps and have better long term stability. On the other
hand, they have a limited number of emission wavelengths, which limits the selection of dyes
that can be employed. The large lasers are not suited for field use, and they are expensive to
buy as well as to operate. Arc lamps can be used for all dyes, they are rugged and
inexpensive. Arc lamps have much shorter life times than lasers. (Typically the nominal life
time is 200 hours for 100 W Hg lamp and 400 hours for a 75 W Xe lamp.) On the other
hand, they are easy to replace and relatively inexpensive.
Can we expect that a light source which combines the best properties of lasers and arc lamps
is going to be developed? The new laser diodes have a stable output with sufficient power,
and they are rugged and inexpensive. However, so far the emission is only in the red and
infrared, i.e. above 650 nm. Although there are now promising developments with regard to
fluorescent dyes which can be excited by these wavelengths, such lasers cannot be used to
excite many of the natural pigments of marine microorganisms. Just as today's laser diodes
were developed primarily to satisfy the producers of mass produced products such as CDplayers, a new generation of laser diodes, with emission in the blue and possibly UV, may
result from efforts to develop more efficient optical memory devices for the computer
industry. Such light sources are sure to have a major impact on flow cytometer design,
performance and price.
Fluorescence detection optics: As evident from Eq. 2, optimal fluorescence detection requires
that the numerical aperture of the light collection optics is as high as possible. In flow
cytometers where the cells are measured in a jet in air, the numerical aperture of the detection
optics is limited by the fact that the lens cannot approach the jet within a certain distance.
Thus, the numerical aperture of the fluorescence collecting lens in such instruments is about
NA = 0.6. The effective aperture is further limited by the obscuration bar, which is a strip
instruments can be eliminated by a monitoring system which measures the intensity and adjusts
the signal amplifier gains so as to be inversely proportional to the intensity (Steen 1980).
Under field conditions, arc lamps have an advantage in their low power consumption and
resistance to mechanical shock and vibration.
Clearly, both lasers and arc lamps have their advantages and disadvantages. Lasers can
produce higher intensities than arc lamps and have better long term stability. On the other
hand, they have a limited number of emission wavelengths, which limits the selection of dyes
that can be employed. The large lasers are not suited for field use, and they are expensive to
buy as well as to operate. Arc lamps can be used for all dyes, they are rugged and
inexpensive. Arc lamps have much shorter life times than lasers. (Typically the nominal life
time is 200 hours for 100 W Hg lamp and 400 hours for a 75 W Xe lamp.) On the other
hand, they are easy to replace and relatively inexpensive.
Can we expect that a light source which combines the best properties of lasers and arc lamps
is going to be developed? The new laser diodes have a stable output with sufficient power,
and they are rugged and inexpensive. However, so far the emission is only in the red and
infrared, i.e. above 650 nm. Although there are now promising developments with regard to
fluorescent dyes which can be excited by these wavelengths, such lasers cannot be used to
excite many of the natural pigments of marine microorganisms. Just as today's laser diodes
were developed primarily to satisfy the producers of mass produced products such as CDplayers, a new generation of laser diodes, with emission in the blue and possibly UV, may
result from efforts to develop more efficient optical memory devices for the computer
industry. Such light sources are sure to have a major impact on flow cytometer design,
performance and price.
Fluorescence detection optics: As evident from Eq. 2, optimal fluorescence detection requires
that the numerical aperture of the light collection optics is as high as possible. In flow
cytometers where the cells are measured in a jet in air, the numerical aperture of the detection
optics is limited by the fact that the lens cannot approach the jet within a certain distance.
Thus, the numerical aperture of the fluorescence collecting lens in such instruments is about
NA = 0.6. The effective aperture is further limited by the obscuration bar, which is a strip