An optical leak is a manufacturing flaw in an instrument that allows light from unwanted sources to illuminate the detectors while they are collecting data. While this can be a major concern on some satellites, the known optical leaks on the Landsat satellites are minor.
Landsat 5 (L5) Thematic Mapper (TM) Internal Calibrator (IC) Light Leak
The Landat 5 Thematic Mapper (TM) light leak is a secondary image next to the IC lamp pulse. It is caused by an incomplete masking of the optical path by the IC shutter. It appears in all reflective bands and only in the IC region; thus, it’s only found in Level-0 data. The TM light leak causes difficulties in using the IC pulse for calibration, but has no other effect on the data.
Figure 1. Landsat 5 TM band 7 IC data showing the IC light leak.
Click to view larger image. - .gif (162 KB)
Landsat 7 Ghost Partial Aperture Solar Calibrator
On Landsat 7, an extra partial aperture solar calibrator (PASC) signal appears in dark data about one Worldwide Reference System (WRS) row behind the first PASC glint. This "ghost PASC" is dimmer than the real PASC glint and is dimmer in some bands due to spectral effects, making it appear blue in Red Green and Blue (RGB) color value images made with band combinations 321 or 543 (browse images). The ghost PASC is caused by unwanted reflections from the sun in the partial aperture solar calibrator optics. It only appears in Landsat 7 reflective band night imagery, during or immediately after PASC collections. The ghost PASC has no effect on calibration. Because it only occurs in reflective bands during PASC collections, any imagery it obscures is nighttime data.
Figure 2. Landsat 7 Enhanced Thematic Mapper Plus (ETM+) browse imagery showing the L7 ghost partial aperture solar calibrator.
Click to view larger image. - .gif (267 KB)