Some error is introduced into the light distribution measurement of a lamp, because the reflection factor of the mirror system used for the measurement is influenced by the states of polarisation of incident light. For high pressure mercury vapour lamps and fluorescent mercury lamps, the states of polarisation of light emitted by them were measured and the following equation for evaluation of the error has been obtained by the theoretical treatment.
Δ_θ=ρ/2 [φ_θcos (α_θ-β_θ)-φ₀cos (α₀-β₀)], where Δ_θ=the error in the relative value of luminous intensity in the direction which makes an angle θ with the normal to the axis of the inner tube of a mercury lamp, ρ=the degree of polarisation concerning the reflection factor of the mirror system, φ_θ =the degree of polarisation of luminous flux in the direction which makes an angle θ with the above normal, φ₀ =the degree of polarisation of luminous flux in the direction of the normal, (α_θ-β_θ) =the phase difference between planes of the polarisation of luminous flux and reflection in the mirror system, in the case of the measurement of luminous intensity in the direction which makes an angle θ with the above normal, (α₀-β₀) =the phase difference between these planes, in the case of the measurement of luminous intensity in the direction of the normal.
Values of Δθ were calculated by the above equation and data concerning polarisation of light emitted by mercury lamps. As the result of it, errors introduced into the total flux of them have been estimated about 0.1-0.2% for general cases.

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We have developed a new type of fluorescent lamp that has an ultraviolet absorbing layer inside.
This new lamp has the following features:
1. It has practically no rays in shorter wavelength than 390 mμ Therefore the intensity of ultraviolet rays of this one is less than that of the standard type one by one hundredth, and that of an incandescent lamp by one twentieth at equal illumination.
2, The luminous flux of this lamp is less than that of the standard type by 5% or so.
3. This Iamp is equivalent to the standard type one in lamp characteristics such as color-rendering, performance, etc.
4. Making this lamp and controlling the thickness of its ultraviolet absorbing layer are not so difficult, and the procedure can be applied to making any kind of fluorescent lamps.
The comparison between this lamp and the standard type one in actinism (i. e. fading) has been done using various dyed textures.
This lamp has a half actinic power of the standard type one. Especially for white or near white textures, it has one fifth actinic power.
This lamp has the same power of actinism as an incandescent lamp for various dyed textures.
This lamp will especially be recommended for lighting in shops, museums and galleries because it prevents color-fading of various goods such as textures, prints, foodstuff, chemicals, arts, etc.

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