colour_hdri.saturation_based_speed_focal_plane_exposure#

colour_hdri.saturation_based_speed_focal_plane_exposure(L: ArrayLike, A: ArrayLike, t: ArrayLike, S: ArrayLike, F: ArrayLike = 50 / 1000, i: ArrayLike = 1 / -1 / 5 + 1 / 50 / 1000, H_f: ArrayLike = 0, T: ArrayLike = 9 / 10, f_v: ArrayLike = 98 / 100, theta: ArrayLike = 10) → NDArrayFloat[source]#

Compute the Saturation-Based Speed (SBS) focal plane exposure \(H_{SBS}\) in lux-seconds (\(lx.s\)).

The model implemented by this definition is appropriate to simulate a physical camera in an offline or realtime renderer.

Parameters:

  • L (ArrayLike) – Scene luminance \(L\), expressed in \(cd/m^2\).

  • A (ArrayLike) – Lens F-Number \(A\).

  • t (ArrayLike) – Exposure Time \(t\), expressed in seconds.

  • S (ArrayLike) – ISO arithmetic speed \(S\).

  • F (ArrayLike) – Lens focal length \(F\), expressed in meters.

  • i (ArrayLike) – Image distance \(i\), expressed in meters.

  • H_f (ArrayLike) – Focal plane flare exposure \(H_f\), expressed in lux-seconds (\(lx.s\)).

  • T (ArrayLike) – Transmission factor of the lens \(T\).

  • f_v (ArrayLike) – Vignetting factor \(f_v\).

  • theta (ArrayLike) – Angle of image point off axis \(\theta\).

Returns:

Saturation-Based Speed focal plane exposure \(H_{SBS}\) in lux-seconds (\(lx.s\)).

Return type:

numpy.ndarray

Notes

  • Focal plane exposure is also named luminous exposure or photometric exposure and is time-integrated illuminance.

  • Object distance \(o\), focal length \(F\), and image distance \(i\) are related by the thin-lens equation: \(\cfrac{1}{f}=\cfrac{1}{o}+\cfrac{1}{i}\)

  • The image distance default value is that of an object located at 5m and imaged with a 50mm lens.

  • The saturation based speed, \(S_{sat}\), of an electronic still picture camera is defined as: \(S_{sat}=\cfrac{78}{H_{sat}}\) where \(H_{sat}\) is the minimum focal plane exposure, expressed in lux-seconds (\(lx.s\)), that produces the maximum valid (not clipped or bloomed) camera output signal. This provides \(1/2\) “stop” of headroom (41% additional headroom) for specular highlights above the signal level that would be obtained from a theoretical 100% reflectance object in the scene, so that a theoretical 141% reflectance object in the scene would produce a focal plane exposure of \(H_{sat}\).

  • The focal plane exposure \(H_{SBS}\) computed by this definition is almost equal to that given by scene luminance \(L\) scaled with the output of colour_hdri.photometric_exposure_scale_factor_Lagarde2014() definition.

References

[ISO06]

Examples

>>> saturation_based_speed_focal_plane_exposure(  
...     4000, 8, 1 / 250, 400, 50 / 1000, 50 / 1000, 0.0015
... )
0.8430446...