Spectroscopic Light Sources

1. Continuum Sources

2. Line Sources

3. Quasi-continuum Sources

Source Types

Source Characteristics

Continuum Sources

Emit radiation over a broad spectral range.

Continuum in Wavelength, not necessarily in time.

Most of these are “black body emitters. The spectral range depends upon the temperature of the black body.

Spectral Radiance of a Black Body

-4

-3

-2

-1

0

1

2

2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7

Log Wavelength (nm)

Lo

g B

(W

cm

-2 s

r-1

nm

-1)

1000

2000

3000

4000

5000

6000

7000

80009000

1000011000

Continuum Sources

Continuum Sources

Tungsten Lamps

1. Absorption Filters

Optical material containing an absorber that permits transmission only at certain wavelengths

Reflection Grating:

Linear DispersionDl

Dl = f × Da = dx/dλ

where:

f = focal length(of monochromator)

x = distance(along focal plane)

Spectral Bandpass(s or Δλ)

s = RLD × w

where:

w = slit width (mm)

s in nm

Reciprocal LinearDispersion (RLD)

RLD = Dl-1

=dλ/dx

nm/mm

3. Si Photodiode

Si: 3s23p2

Covalent Bonds in Solid

Therefore 1/2–filledsp3

ΔE ≤ 2.5 eV(semiconductor)

4 electrons fill a valence band at 0KAt higher T an electron can move to conduction bandLeaving a positive hole behind (both are mobile)

3. Si Photodiode

Doping Si with a group 5element (As or Sb) resultsin extra electrons (n-type).

Doping with a group 3element (In, Ga) resultsin extra holes (p-type)

3. Si Photodiode

3. Si Photodiode

Forward bias (not very useful for spectroscopy)

3. Si Photodiode

Reversed Bias:

Depletion zone at thejunction.

Photons may eject electrons and formholes

Current proportionalto number of photons

3. Si Photodiode

200 – 1000 nm

1-10 ns response time

0.05 A/W

4. Linear Photodiode Array

200 – 1000 nm

1-10 ns response time

0.05 A/W

4. Linear Photodiode Array