At sample times (also known as integration time) greater than 1000 milliseconds, it is impossible to differentiate between the normal signal noise of the spectrometer and a measurable reading of light emission. The readings have no significant meaning. This makes it difficult to measure the light emission from very faint source, such as: glowing solids or liquids, reflected light, or celestial objects.
This is only an issue when measuring emissions. When you use a spectrometer to measure absorbance or % transmittance, the sample time is automatically determined by the software during calibration.