Visible

Analysis & Data Products

Our visible analysis algorithms use modern techniques and the latest camera technologies to deliver data products unparalleled in all-sky imaging systems.

Sky cover percentage

Sky cover is computed in two user-defined fields of view (typically 100° and 160°). Three categories — clear, thin, and opaque — are defined by user-configured thresholds applied to processed Red/Blue (R/B) and Green/Blue (G/B) ratio images.

A parameterization of the clear-sky R/B and G/B ratios as a function of airmass is subtracted from the raw ratio images. A secondary subtraction removes the sun’s R/B and G/B contribution as a function of angular distance from the sun. Sky cover thresholds are then applied to these processed images.

Visible image and Red/Blue Ratio Mask — **Figure 1** — Left: visible image. Right: Red/Blue Ratio Mask showing sky cover assessment (Blue = clear, Gray = thin, White = opaque).

Airmass diagnostic plots

R/B and G/B ratio data plotted as a function of airmass are used to assess the quality of selected clear-sky parameters. On-the-fly parameterization of these data assists in refining clear-sky parameter selection.

R/B ratio vs airmass plot — **Figure 2** — R/B ratio pixel data vs. airmass for the image in Figure 1. The manual fit (green line) uses stored library parameters for the clear-sky R/B ratio radiance. On-the-fly parameters are computed per image using the lowest R/B ratio data in specific airmass bins.

Residual R/B ratio after clear-sky subtraction — **Figure 3** — Same data as Figure 2 after subtracting the clear-sky R/B ratio using the manual fit. Threshold values (thin and opaque) are applied to produce the cloud decision map shown in Figure 1.

Diagnostic sun profile plots

R/B and G/B ratio data are plotted as a function of pixel distance from the sun. This analysis improves cloud/no-cloud detection and informs selection of the sun exclusion mask in the vicinity of the sun.

Exposure time monitoring

Exposure time monitoring enables conversion of all RGB channel data to instrumental (uncalibrated) intensity. Combined with ancillary calibration data, this supports additional data products such as downwelling radiance and atmospheric visibility.