Close-up lenses for model S3 and S4
Measuring the luminance of small surfaces.
The Hagner Universal Photometer can be provided with close-up lenses for measuring the luminance of small surfaces at short distances, for example text on data terminal screens, without troublesome light loss.
The close-up lenses are available in sets of 3:
- Focal length 500 mm: (2 dioptres)
- Focal length 250 mm: (4 dioptres)
- Focal length 125 mm: (8 dioptres)
The lenses can be used separately or in combination with each other.
The following table applies when using close-up lenses with the Hagner Universal Photometer.
| Lens combination | Focal length (mm) | Measuring distance* | Object-lens approx mm |
| A | 500 | 500 | 1.11 |
| B | 250 | 250 | 1.11 |
| C | 125 | 130 | 1.11 |
| A + B | 167 | 170 | 1.23 |
| A + C | 100 | 100 | 1.23 |
| B + C | 83 | 85 | 1.23 |
*The measuring distance is equal to the distance when the object is seen sharply through the view-finder. Lense combination is counted from the instrument and outwards.
When close-up lenses are used, the lumiance (L) of the object is obtained from the following equation:
L = F x LM where F is the correction factor from the above table and LM is the luminance measured with the close-up lense mounted.
It should be noted that the luminance (L) is the average luminance of a cone with the object on the "top" and the surface of the outermost lens as a base.