The approximation on the right-hand side of Eq. (4.3) only holds for small A
and smallΩwhere coshis approximately constant.
Often it is necessary to designate the above quantities as a function of wave-
length in order to correlate the light characteristics to the responses of various tissue
types to particular spectral bands. This designation yields the following parameters:
- Spectralfluxis the radiantflux (power) per wavelength interval
- Spectral irradianceis theflux density (power density) per wavelength interval
- Spectral radiant intensitygives the radiant intensity as a function of wavelength
- Spectral radiant exposure gives the radiant exposure as a function of
wavelength - Spectral radiancespecifies the radiance as a function of wavelength.
4.2 Arc Lamps
Lamps used for scientific and medical biophotonics applications are mainly
high-pressure gas-discharge lamps orarc lamps. Such lamps are based on the
creation of an intense light when an electrical discharge passes through a
high-pressure gas or a vapor. When the lamp is in operation, the pressure is on the
order of 10–40 MPa, where a pascal (Pa) unit is one newton per meter squared
(N/m^2 ). For comparison purposes, the atmospheric pressure at sea level is about
101 kPa. Because the arc length usually is only a few millimeters, these lamps are
known asshort-arc lamps[ 4 – 6 ].
In general the discharge is sustained by the ionization of mercury (Hg) vapor or of
inert gases like xenon (Xe), argon (Ar), and neon (Ne). These light sources come in
many sizes with a wide range of output characteristics. The outputs can be sharp
spectral lines, narrow spectral bands, or wide spectral bands ranging from the ultra-
violet to the infrared regions. Some sources operate in a continuous wave (CW) mode
and others are used in a pulsed mode. The lamps can be used for microscopy, endo-
scopy, minimally invasive surgery, and medicalfiber optic applications.
High-pressure mercury arc lamps have a high-intensity line-type spectral output
in the UV and visible regions as Fig.4.4shows. The spectral lines are mainly in the
300 – 600 nm region. The output powers can range from 50 to 500 W with nominal
focused beam sizes of 5 mm. In general the mercury sources are useful when
certain wavelengths in the line spectra are suitable for monochromatic irradiation
forfluorescence spectroscopic applications.
High-pressure xenon arc lamps are popular and highly versatile radiation sources.
As Fig.4.5shows, these lamps emit an intense quasi-continuous spectrum ranging
from about 250 nm in the UV region to greater than 1000 nm in the near infrared.
The output is relatively continuous from 300 to 700 nm and has a number of sharp
lines near 475 nm and above 800 nm. The emission of Xe lamps is about 95 %
similar to daylight. The lamps are made of two tungsten electrodes encapsulated in a
4.1 Radiometry 97