Optical detection system calibration

1. A method of measuring an optical reflectance R of a target using a detection system comprising a light emitter and a light detector spaced apart from one another, the method comprising:

illuminating the target with the light emitter;

detecting light reflected from the target using the light detector, wherein the light detector provides an electrical output signal S S indicative of the intensity of the detected light; and

determining the optical reflectance R of the target according to

R

=

M

·

R

R

⁢

S

S

-

S

H

M

·

[

S

R

-

S

H

]

-

R

R

[

S

R

-

S

S

]

,

where

R R is the spectral reflectance of a reference standard,

S R is the detector electrical output signal with the reference standard in place,

S H is the detector electrical output signal with no target in front of the light emitter and light detector, and

M is a calibration factor.

2. The method according to claim 1 and comprising:

determining a height z of the target above the detection system;

using said height z to determine a height scaling factor η(z);

scaling the determined reflectance using the scaling factor according to

R

=

M

·

R

R

⁢

S

S

-

S

H

M

·

[

S

R

-

S

H

]

-

R

R

[

S

R

-

S

S

]

·

η

⁡

(

z

)

.

3. The method according to claim 1 , wherein said reflectance R is measured for one or more wavelengths λ, according to:

R

⁡

(

λ

)

=

M

⁡

(

λ

)

·

R

R

(

λ

)

⁢

S

S

(

λ

)

-

S

H

(

λ

)

M

⁡

(

λ

)

·

[

S

R

(

λ

)

-

S

H

(

λ

)

]

-

R

R

(

λ

)

[

S

R

(

λ

)

-

S

S

(

λ

)

]

.

4. The method according to claim 1 , wherein said electrical output signals are integrated over a wavelength range, and said reflectances represent reflectances averaged over the wavelength range.

5. The method according to claim 1 , said step of determining the optical reflectance R of the target comprising obtaining from a memory values for each of R R , S R , S H and M, and evaluating the equation

R

=

M

·

R

R

⁢

S

S

-

S

H

M

·

[

S

R

-

S

H

]

-

R

R

[

S

R

-

S

S

]

.

6. The method according to claim 1 , wherein said step of determining the optical reflectance R of the target comprises using said output signal S S as a look-up to a look-up table, the look-up table being populated with reflectance values evaluated using the equation

R

=

M

·

R

R

⁢

S

S

-

S

H

M

·

[

S

R

-

S

H

]

-

R

R

[

S

R

-

S

S

]

and values for each of R R , S R , S H and M.

7. The method according to claim 1 , wherein the calibration factor M is dependent upon a target material type.

8. A method of obtaining calibration data for the detection system configured to use the method of claim 1 to measure optical reflectance, the method comprising:

operating a calibration detection system comprising a light emitter and a light detector spaced apart from one another, for a plurality of targets of known optical reflectances R and obtaining the respective electrical output signals of the detector S R ; and

solving the following equation to determine a calibration factor M;

S

R

=

k

·

I

0

·

R

M

⁢

1

1

-

R

M

,

where k is a multiplicative factor taking into account the responsivity of the photodetector, potential filter responses and/or interposed media,

where I 0 is the light intensity of the light emitter.

9. The method according to claim 8 , wherein:

said calibration detection system is the detection system for which calibration data is being obtained, or

said calibration detection system is a different system from the detection system for which calibration data is being obtained and the method comprises providing the calibration factor M to the detection system for which calibration data is being obtained.

10. The method according to claim 8 , wherein the detection system for which calibration data is being obtained is configured to measure said reflectance R for one or more wavelengths λ, according to:

R

⁡

(

λ

)

=

M

⁡

(

λ

)

·

R

R

(

λ

)

⁢

S

S

(

λ

)

-

S

H

(

λ

)

M

⁡

(

λ

)

·

[

S

R

(

λ

)

-

S

H

(

λ

)

]

-

R

R

(

λ

)

[

(

S

R

(

λ

)

-

S

S

(

λ

)

]

,

the method comprising performing the steps of operating and solving for one or more wavelengths, and determining a value of M(λ) for each wavelength.

11. The method according to claim 8 , wherein the detection system is configured to:

determine a height z of the target above the detection system;

use said height z to determine a height scaling factor η(z);

scale the determined reflectance using the scaling factor according to

R

=

M

·

R

R

⁢

S

S

-

S

H

M

·

[

S

R

-

S

H

]

-

R

R

[

S

R

-

S

S

]

·

η

⁡

(

z

)

,

and comprising repeating the steps of operating and solving, for a plurality of different heights z above the calibration detection system, to determine values of M and η(z) for respective heights.

12. An optical reflectance measurement system comprising a chassis and, mounted to the chassis, a light emitter and a light detector, the system further comprising a processor or processors configured to:

cause the light emitter to illuminate the target;

cause the light detector to detect light reflected from the target, wherein the light detector provides an electrical output signal S S indicative of the intensity of the detected light; and

determine an optical reflectance R of the target according to

R

=

M

·

R

R

⁢

S

S

-

S

H

M

·

[

S

R

-

S

H

]

-

R

R

[

S

R

-

S

S

]

,

where

R R is the spectral reflectance of a reference standard,

S R is the detector electrical output signal with the reference standard in place,

S H is the detector electrical output signal with no target in front of the light emitter and light detector, and

M is a calibration factor.

13. The optical reflectance measurement system according to claim 12 and comprising a proximity sensor for determining a height z of the target above the detection system, said processor or processors configured to:

use said height z to determine a height scaling factor η(z);

scale the determined reflectance using the scaling factor according to

R

=

M

·

R

R

⁢

S

S

-

S

H

M

·

[

S

R

-

S

H

]

-

R

R

[

S

R

-

S

S

]

·

η

⁡

(

z

)

.

14. The optical reflectance measurement system according to claim 12 , said processor or processors configured to determine said reflectance R for one or more wavelengths λ, according to:

R

⁡

(

λ

)

=

M

⁡

(

λ

)

·

R

R

(

λ

)

⁢

S

S

(

λ

)

-

S

H

(

λ

)

M

⁡

(

λ

)

·

[

S

R

(

λ

)

-

S

H

(

λ

)

]

-

R

R

(

λ

)

[

S

R

(

λ

)

-

S

S

(

λ

)

]

.

15. The optical reflectance measurement system according to claim 12 and comprising a memory, wherein, either,

said processor or processors is configured to obtain from said memory, values for each of R R , S R , S H and M, and to evaluate the equation

R

=

M

·

R

R

⁢

S

S

-

S

H

M

·

[

S

R

-

S

H

]

-

R

R

[

S

R

-

S

S

]

.

,

or

said processor or processors is configured to use said output signal S S as a look-up to a look-up table stored in said memory, the look-up table being populated with reflectance values evaluated using the equation

R

=

M

·

R

R

⁢

S

S

-

S

H

M

·

[

S

R

-

S

H

]

-

R

R

[

S

R

-

S

S

]

and values for each R R , S R , S H and M.