Where vision is concerned, light is generally specified in photometric units, not in quanta (photons) and energy

Commission Internationale de l’Eclairage (CIE)

Fig. A-1. The photopic (V ) and scotopic (V ’)curves of relative luminosity as standardized by the Commission Internationale de l’Eclairage (CIE). Modified from Wright (1958)

Just because something is twice as intense does not mean it is twice as bright!!

Photometric Terminology Point Source Luminous flux is emitted in all directions from a point source of light. A lumen is equivalent to 4.07 x quanta/ second at 555 nm. Luminous intensity is luminous flux in a solid angle. A candela is defined as one lumen/ steradian. Luminance is the light that comes off a surface whether reflected or emitted. It is measured in candelas/unit area iIluminance is what falls on a surface. It is measured in lumens/ unit area  r B A

A steradian is "the solid angle subtended at the center of a sphere of radius r by a portion of the surface of the sphere having an area r2."

Fig. A.2 Point Source Luminous flux is emitted in all directions from a point source of light. A lumen is equivalent to 4.07 x quanta/ second at 555 nm. Luminous intensity is luminous flux in a solid angle. A candela is defined as one lumen/ steradian. Luminance is the light that comes off a surface whether reflected or emitted. It is measured in candelas/unit area iIluminance is what falls on a surface. It is measured in lumens/ unit area  r B A

Point Source Luminous flux is emitted in all directions from a point source of light. A lumen is equivalent to 4.07 x quanta/ second at 555 nm. Luminous intensity is luminous flux in a solid angle. A candela is defined as one lumen/ steradian. Luminance is the light that comes off a surface whether reflected or emitted. It is measured in candelas/unit area iIluminance is what falls on a surface. It is measured in lumens/ unit area  r B A

Point Source Luminous flux is emitted in all directions from a point source of light. A lumen is equivalent to 4.07 x quanta/ second at 555 nm. Luminous intensity is luminous flux in a solid angle. A candela is defined as one lumen/ steradian. Luminance is the light that comes off a surface whether reflected or emitted. It is measured in candelas/unit area iIluminance is what falls on a surface. It is measured in lumens/ unit area  r B A

Point Source Luminous flux is emitted in all directions from a point source of light. A lumen is equivalent to 4.07 x quanta/ second at 555 nm. Luminous intensity is luminous flux in a solid angle. A candela is defined as one lumen/ steradian. Luminance is the light that comes off a surface whether reflected or emitted. It is measured in candelas/unit area iIluminance is what falls on a surface. It is measured in lumens/ unit area  r B A

Point Source Luminous flux is emitted in all directions from a point source of light. A lumen is equivalent to 4.07 x quanta/ second at 555 nm. Luminous intensity is luminous flux in a solid angle. A candela is defined as one lumen/ steradian. Luminance is the light that comes off a surface whether reflected or emitted. It is measured in candelas/unit area iIluminance is what falls on a surface. It is measured in lumens/ unit area  r B A

Photopic Scotopic Mesopic

Contrast is not a photometric term.Contrast is an expression of luminous difference between two surfaces. The standard quantitative definition of contrast for a target on a background is: BBT /L)L(L  Eq. A.4 where L B is the luminance of the reference surface and L T is the luminance of the second surface. If L T > L B then the contrast is positive; otherwise it is negative. This is sometimes called “Weber Contrast”

The contrast of sine-wave gratings is measured differently Fig. 6.6

Sine-wave gratings are measured in terms of their spatial frequency defined as the number of cycles per degree of visual angle Fig. 6.7

Specifying and Using Visual Angle Stimulus size is often expressed in terms of visual angle Fig. A.3

Specifying and Using Visual Angle Stimulus size is often expressed in terms of visual angle Two advantages: 1)Provides a measure of the stimulus size on the retina 2)That allows investigators in other labs to duplicate the stimulus size (without needing to duplicate the equipment)

Objects A and B are the same size, but subtend different angles on the retina because they are at different distances from the cornea Object C subtends the same angle as object A, so A and C would be indistinguishable based on retinal size and position alone Can calculate the visual angles subtended by the visual stimuli:

Visual angles are expressed in degrees, minutes or seconds of arc Large stimuli subtend visual angles expressed in degrees (  ) (e.g., a 10  spot) There are 360  in a circle Smaller stimuli are described in minutes (’) of arc There are 60’ in 1  of arc Still smaller stimuli are expressed in second (”) of arc There are 60” in 1’ of arc