PAR irradiance from a spectral irradiance measurement.

Integrates spectral irradiance over the photosynthetically active range 400–700 nm and returns the result in µmol photons m$^{-2}$ s$^{-1}$. Accepts energy units (W m$^{-2}$ nm$^{-1}$), raw photon counts, or a lux_spectrum object in any supported unit.

Usage

par_irradiance(irradiance, ...)

# S3 method for class 'numeric'
par_irradiance(irradiance, lambda, photonic = FALSE, binwidth = NULL, ...)

# S3 method for class 'lux_spectrum'
par_irradiance(irradiance, ...)

Arguments

irradiance: Spectral irradiance (W m$^{-2}$ nm$^{-1}$ or photons s$^{-1}$ m$^{-2}$ nm$^{-1}$) at each bin, or a lux_spectrum object.
...: Ignored.
lambda: Wavelength vector in nm (not needed for lux_spectrum).
photonic: Logical. If FALSE (default), input is in W m$^{-2}$ nm$^{-1}$. If TRUE, input is in raw photon counts (photons m$^{-2}$ s$^{-1}$ nm$^{-1}$), not molar units. For molar-unit (µmol/m²/s/nm) input, either wrap in a lux_spectrum or convert to W/m²/nm first with n2W_spec_irradiance. Note: photonic = TRUE means molar units in n2W_spec_irradiance and photon-or-molar units in irradiance2lux.
binwidth: Bin width in nm. Inferred from lambda if NULL.

Value

Scalar PAR irradiance in µmol photons m$^{-2}$ s$^{-1}$.

Details

PAR is the photon flux integrated over the photosynthetically active range 400–700 nm, expressed in µmol photons m$^{-2}$ s$^{-1}$. When input is in energy units (W m$^{-2}$ nm$^{-1}$), each bin is first converted to photon flux via $N = E\lambda/(hc)$, then the total is divided by Avogadro's constant scaled to µmol: $$Q_{\mathrm{PAR}} = \frac{1}{N_A \times 10^{-6}} \int_{400}^{700} \frac{E(\lambda)\,\lambda}{hc}\,d\lambda$$ where $N_A = 6.022 \times 10^{23}$ mol$^{-1}$ and $hc = 1.98645 \times 10^{-16}$ J nm. The result is in µmol photons m$^{-2}$ s$^{-1}$.

References

Thimijan RW, Heins RD (1983) HortScience 18:818-822.

Examples

sp  <- solar_irradiance("clear_noon")
par_irradiance(sp$irradiance, sp$wavelength)
#> [1] 2197.756