A. Errors evidenced by inconstant operation of single instruments |
(from characteristics of the photo-cells) | | |
1. | Change of settings with temperature of instrument. | Change of photo-cell current with temperature. | Allow instrument to warm up to equilibrium and check with standard frequently to detect drift. |
2. | Galvanometer drift after opening shutter. | Change with time, of current from one photo-cell relative to that from the other (fatigue). | Use photo-cells paired for equal rate of drift and thus eliminate residual drift. |
3. | Galvanometer response to change of line voltage. | Change with source intensity of current from one photo-cell relative to that from the other. | Use photo-cells paired for like changes of current with identical changes of illumination. |
4. | Persistent failure of instrument to reproduce readings made shortly before, galvanometer drift. | Failure of contacts within photo-cells. | Renew electrical contacts within cells, or replace cells. |
(from design of the instrument) | | |
5. | Galvanometer response to movement of a light past cracks and openings in the instrument. | Light leakage. | Make housing light tight, or use instrument in a darkened room. |
6. | Galvanometer response to pressure on, or movement of instrument during a setting. | Lack of structural rigidity. | Build instrument rigidly, handle carefully during use. |
7. | Erratic operation. | Dirt on optical parts. | Clean parts affected. |
(from characteristics of the sample) | | |
8. | Galvanometer response to change of line voltage only when instrument beams have markedly different spectral composition. | Marked spectral difference of samples in two beams. | Operate source at constant voltage when beams have markedly different spectral composition. |
9. | Change of instrument reading with change of area of surface exposed. | Non-uniformity of surface of the sample in the quality being measured. | Designate area of surface measured. |
10. | Change of instrument reading with wobble of surface over opening. | Nonflatness of surface of sample. | Flatten surface or designate exact position during measurement. |
11. | Change of instrument reading with rotation of sample surface. | Variation of apparent reflectance of surface with rotation in its own plane. | Make and average four readings of sample rotating it 90° between each pair of readings. |
12. | Change of reading with exposure of sample to light, time, humidity or other treatment. | Impermanence of characteristics of sample. | Treat sample so affected with care and give history prior to measurement. |
B. Errors identified by use of separate instruments |
13. | Failure of reflectometer to check accurate results from separate instruments. | Inaccurate scales. | Prepare and use scale-correction curves following instructions. |
14. | ditto | Inaccurate calibration or change of standard. | Remeasure standard. |
15. | ditto (Frequently appears when results are compared with those from instrument of different type.) | Inexact duplication of specified directional conditions of measurement. | Examine angular conditions of measurement of both instruments involved. (Do not expect accurate duplication of results if there are differences.) |
16. | ditto (Penetration of light is often visible at edges of sample.) | Loss of light by lateral passage after penetration into translucent sample. | Measure on instrument having negligible penetration loss (see text). |
17. | ditto | Failure of spectral response of source-filter-photo-cell combinations to duplicate those required. | Use standards spectrally similar to samples measured. |
18. | ditto | Fluorescence of samples. | Samples must be measured on instrument not subject to fluorescence error. |