An illuminant is characterized by its relative spectral power distribution (SPD). Filters are sometimes added to the UV illumination source to narrow the illuminant waveband. The surround light should be 5 cd/m and be neutral grey at standard illuminant D. This differs from the CCT of the illuminant (D65). Illuminant and white point are separate concepts. This was the last Mk 8 illuminant fired in conflict. The set of three tristimulus coordinates of an illuminant is called a "white point". They modify Illuminant A by using liquid filters. Manufacturers sometimes compare light sources against Illuminant E to calculate the excitation purity. Beyond Happisburgh, however, Trinity House did not adopt gas as an illuminant for its lighthouses. It can also be used in textile manufacture, soap making and as an illuminant. Illuminant E is an equal-energy radiator; it has a constant SPD inside the visible spectrum. For a given illuminant, its white point is uniquely defined. Around 1848, Samuel Kier realized the potential of the medicinal oil as an illuminant.