If the quantity of x does not start with 'energy'
then the quantity is not radiometric and so x is returned unchanged.
Otherwise x is radiometric (energy-based), and must be converted.
If type(x) is 'light' then
the most common radiometric energy unit is joule.
The conversion equation is:
$$ Q = E * \lambda * 10^6 / (N_A * h * c)$$
wher \(Q\) is the photon count,
\(E\) is the energy of the photons,
\(N_A\) is Avogadro's constant,
\(h\) is Planck's constant, \(c\) is the speed of light,
and \(\lambda\) is the wavelength.
The output unit of photon count is
(\(\mu\)mole of photons) = (\(6.02214 * 10^{17}\) photons).
If a different unit for Q is desired,
then the output should be scaled appropriately.
For example, if the desired unit of photon count is exaphotons,
then set multiplier=0.602214.
If the quantity(x) is 'energy->electrical',
then the most common radiometric unit of responsivity to light
is coulombs/joule (C/J) or amps/watt (A/W).
The conversion equation is:
$$ QE = R_e * ((h * c)/e) / \lambda $$
where \(QE\) is the quantum efficiency,
\(R_e\) is the energy-based responsivity,
and \(e\) is the charge of an electron (in C).
If the unit of x is not C/J,
then multiplier should be set appropriately.
If the quantity(x) is
'energy->neural' or 'energy->action',
the most common radiometric unit of energy is joule (J).
The conversion equation is:
$$ R_p = R_e * 10^{-6} * ( N_A * h * c) / \lambda $$
where \(R_p\) is the photon-based responsivity,
and \(R_e\) is the energy-based responsivity,
The output unit of photon count is
(\(\mu\)mole of photons) = (\(6.02214 * 10^{17}\) photons).
This essentially the reciprocal of the first conversion equation.
The argument multiplier is applied to the right side of all the above
conversion equations.