The Patent

Definitions

visibility

/vɪzɪˈbɪlɪti/
  • the state of being able to see or be seen.
“a reduction in police presence and visibility on the streets”
  • the distance one can see as determined by light and weather conditions.
“visibility was down to 15 yards”
  • the degree to which something has attracted general attention; prominence.
“the issue began to lose its visibility”
  • the greatest safe distance at which a black object of suitable dimensions, situated near the ground, can be seen and recognised when observed against a bright background;
  • the greatest distance at which lights in the vicinity of 1000 candelas can be seen and identified against an unlit black background.

General considerations

Visibility was first defined for meteorological purposes as a quantity to be estimated by a human observer, and observations made in that way are widely used. However, the estimation of visibility is affected by many subjective and physical factors. The essential meteorological quantity, which is the transparency of the atmosphere, can be measured objectively and is represented by the meteorological optical range (MOR). [1]

The meteorological optical range is the length of path in the atmosphere required to reduce the luminous flux in a collimated beam from an incandescent lamp, at a colour temperature of 2 700 K, to 5% of its original value, the luminous flux being evaluated by means of the photometric luminosity function of the International Commission on Illumination (CIE).

Visibility, meteorological visibility (by day) and meteorological visibility at night are defined as the greatest distance at which a black object of suitable dimensions (located on the ground) can be seen and recognized when observed against the horizon sky during daylight or could be seen and recognized during the night if the general illumination were raised to the normal daylight level.

Visual range (meteorological): Distance at which the contrast of a given object with respect to its background is just equal to the contrast threshold of an observer

State-of-art

 
MOR visibility is presently determined by means of transmissometers, Lidars or scatter meters. These technologies have crucial limitations: transmissometers are affected by alignment issues and require frequent calibrations; Lidars have high costs, thus hindering their large-scale use; finally, scatter meters are affected by similar issues, including cost.

Transmissiometers make a direct observation of the area by using two collimated lasers and a receiver positioned at some distance away . They are ideal for capturing key visibility information like runway visual range (RVR) at an airport.

Scatter meters: in these instruments, one or more sensors measure the light produced by a source and diffused by the atmosphere, then relating the scattering with the absorption coefficient and therefore with the MOR. The principle of operation is the same as for Lidars but the  difference, however, concerns the position of the sensors : in forward scatter instruments, those are placed off the beam axis and observed with an angle between 20° and 50°.

LiDAR Light Detection and Ranging: here the emitting units fire hundreds of thousands of pulses per second. These light waves bounce off objects and return to the LiDAR sensor. The sensor uses the time it takes for each pulse to return to calculate distance (time of flight). Each of these pulsed laser measurements, or returns, can be processed into matrix points.

INSPICIO patent

The international reference document for the visibility measurement is ” Guide to Meteorological Instruments and Methods of Observation ” (hereinafter GMIMO) published in 2014 by the ” World Meteorological Organization ” (WMO) and updated in 2017.

Among the various quantities defined in this document, the quantity MOR is defined as follows: “MOR, Meteorological Optical Range: length of path in the atmosphere required to reduce the luminous flux in a collimated beam generated by an incandescent lamp at a temperature of 2700 K, at 0.05 times its original value”

The definition of the quantity MOR is based on a physically significant measurement. By applying the Lambert-Beer’s law (named Bouguer- Lambert’s in the GMIMO publication), one has:

\(F(x)=F_{0}e^{-x\sigma } \)

where \(F_{0}\) is the luminous flux at \(x=0\), \(F(x)\) that in \(x\), and \(0\) is defined as extinction coefficient. The value of \(x\) is thus given by:

\(x = -\frac{1}{\sigma }ln\frac{F(x)}{F_{0}}\)

By definition

\(MOR=x \Leftrightarrow \frac{F(x)}{F_{0}} = 0.05\)

In the GMIMO publication, the ratio \(F(x)/F0\) is defined as transmission factor \(T \) (and the quantity MOR is given an additional symbol, \(P\)). Therefore it must result:

\(MOR = -\frac{1}{\sigma }ln(0.05)=\frac{ln(20)}{\sigma }\)

In conclusion, if the transmission factor \(T\) is measured on a reference distance (baseline) \(b\), the quantity MOR is equal to:

\(MOR=-b\frac{ln(20))}{ln(T)}\)

INSPICIO application

One camera. Two mirrors.

The first mirror faces both the camera and the second mirror. The second mirror is adjacent to the camera and faces, parallel to it, the first one.

The camera “sees” its own image reflected by the first mirror and, as a result of a triple (two-plus-one) reflection by both mirrors, a second image, which is two times farther and smaller than the first one.

Once corrected for the mirrors’ finite reflectivity, the contrasts of the two images are equal in the case of a perfectly clean atmosphere. On the other hand, the denser the fog, dust, pollutants between camera and mirrors, the smaller the ratio between the two image contrasts.

A measurement of the ratio directly leads to the assessment of visibility.

[1] WMO, Part I. Measurement of meteorological variables. Chapter 9, Definitions