Si1120

Rev. 1.0

11

2.4. Ambient-Light Modes

Proximity offset and gain can be affected a few percent by high ambient light levels (e.g. sunlight or strong

incandescent lighting). While the cal mode can be used to determine offsets from large ambient light or ambient

noise levels in PRX400 and PRX50 modes, direct measurement of the ambient levels can help identify whether

changes in reflectance are valid or in fact due to large ambient light changes. Usually, this is only an issue in high

reflectance situations, such as single window operation without good optical isolation, where large changing

ambients are an issue.

The Si1120 has two photodiodes, each of which peaks at a different wavelength. The VAMB mode uses the visible

light photodiode which peaks at around 530 nm. On the other hand, the VIRH and VIRL modes use the photodiode

which peaks at around 830 nm. Although the visible-light photodiode peaks near 550 nm (considered the peak

wavelength of human perception), the Si1120 visible photodiode extends to infrared light as well. Similarly, the

Si1120 infrared photodiode detects infrared light as well as part of the visible light spectrum. The Si1120 treats

ultraviolet, visible, and infrared light as a continuous spectrum.

The ratio between the visible and infrared photodiode readings provides a good clue to the type of light source. The

reason is that each light source consists of a characteristic mix of infrared and visible light. For example, blackbody

radiators, such as incandescent or halogen lamps, can have significant energy in the infrared spectrum. On the

other hand, fluorescent lamps have more energy in the visible light spectrum. The term “color ratio” will be used to

describe the relative strength of the visible photodiode reading relative to the infrared photodiode reading. Human

color vision employs a similar principle.

The VAMB/VIRH or VAMB/VIRL color ratios are representative of the Si1120's color perception. Choosing between

these two color ratios depends on the light intensity. In general, VAMB/VIRL should be used first since VIRL has

higher sensitivity. For higher light intensities, the VAMB/VIRH ratio should be used.

Note that VAMB, VIRH, and VIRL pulse widths are used as dividends and divisors in these ratios. What this means

is that the pulse width offsets (at 0 lux) need to be removed prior to usage in the above color ratios. For best

precision, it is best to take VAMB, VIRH, and VIRL measurements at 0 lux and to use actual measured values.

However, a good rule of thumb is to subtract 7.1 µs, 11.3 µs, and 9.9 µs respectively from VAMB, VIRH, and VIRL

(then assigning 0 µs to any resulting negative value). This rule-of-thumb can be used when accuracy is less critical.

Unless stated otherwise, the plots and figures used in this data sheet use offset-corrected values for VAMB, VIRH,

and VIRL.

Once a color ratio has been derived, the light type(s) and lux ratios are also identified. The lux ratio describes the

ratio between the desired lux value and VAMB, VIRL, or VIRH (depending on the situation). The appropriate lux

ratio, when multiplied with the applicable measurement, yields the final calculated lux value. Without any

calibration, it should be possible to arrive within 50% (or 50 lux) of the absolute lux value.

Figure 8. Ambient Light Mode Timing Diagram