(Note: Astronomers use mostly c.g.s)

for frequency, a wide variety used
    2n = c frequency defines
l
    e
= hn frequency defines energy
frequency 1 Hz cycles per second, only frequency used
                                    MHz, GHz (radio)

            but e ~ n and ~ n

Some other interchangeable units

Energy:

1 eV                     1eV -> n1eV = = 2.42 x 1014 Hz (OPTICAL)

1keV            1000eV -> 2.42 x 1017 Hz

1cm-1             = 3 x 1010 cm s-1

Wavelength

1                   1 cm 1 =>12.398 keV
                                        optical, UV, X-ray

1mm                  1mm = 10-4 cm ,   IR

1mm                  1mm = 0.1 cm, Microwaves

1cm                 Radio

wpe2.gif (1481 bytes)

Objects emit photons.      Look at a small area dA, emits N photons of  Frequency g <->n+dA per second

 

wpe3.gif (1393 bytes)dW steradians

n of these go into dW

counted n photons per dA per dn per dW per sec

              or n photons

 

This unit is called "photon intensity"

Convert to "specific intensity" by multiplying by hn

I has units of ergs cm-2 s-1 Hz -1 ster -1

 

"Flux density" is obtained when angle dependence is removed, at dA count all photons emited

1 Jansky = 1 Jy = 10-23 erg cm-2 s-1 Hz-1

Flux = F = -> erg cm-2 s-1 emitted

Brightness = -> erg cm-2 s-1 ster -1