ND400 absorptive neutral density filters have an optical density of 4.0, meaning that they transmit an average of 0.01% of visible light. Surprisingly, this amount of light passing through the filter is easily detectable by the human eye. Because the glass substrate is absorbing 99.99% of the light that passes into the filter, it is not recommended that these filters be used in laser applications or immediately in front of high wattage lamps.