Prism design for scanning applications and illumination of microscopy sample
A prism for use in scanning applications such as total internal reflection microscopy in which the prism is translated relative to an incident light beam. A geometry is disclosed which cancels walk of the beam footprint at the base of the prism. Walk of the beam footprint due to irregularities in a largely planar sample surface located at the prism base are cancelled by coupling movement of the incident light beam to movement of the sample in the field of view of an objective lens, for example as part of an autofocus arrangement.
1. A method for controlling the movement of a light beam in a scanning apparatus, as a function of variation in the length of the optical path of said beam, comprising
(a) directing said light beam to an angular prism, wherein said angular prism is coupled to a sample at a base interface;
characterised in that the base angle θ p of the prism satisfies the equations:
(
cos
2
θ
c
-
sin
2
θ
p
)
(
tan
θ
i
sin
θ
p
+
cos
θ
p
)
=
1
cos
θ
c
cos
(
θ
p
±
θ
c
)
and
n
i
sin
θ
i
=
n
p
sin
(
θ
p
-
θ
c
)
wherein θ c is the coupling angle required for light incident at base interface of the prism, θ i is the incident angle of the light beam on the prism, n i is the refractive index of the medium at the interface where the light beam enters the prism and n p is the refractive index of the prism; and
(b) moving the prism and sample relative to the light beam, wherein said variation in the length of said optical path of said light beam is controlled by aligning the prism so that said light beam enters said prism normal to the face of entry thereinto, and
wherein said light beam and said prism satisfy said equations.
2. The method according to claim 1 , wherein
θ p =θ c and θ i =0.
3. The method according to claim 1 , wherein said apparatus is a total internal reflection microscopy apparatus and includes means for detecting interaction of the sample at the base of the prism with an evanescent wave formed by total internal reflection of the light beam at the base of the prism.
4. The method according to claim 3 , wherein
θ c ≧sin −1 ( n s /n p )
and wherein n s is the refractive index of the sample medium.
5. The method according to claim 1 , wherein said method comprises a method of total internal reflection microscopy, and further comprises the step of detecting interaction of a sample at the base of the prism with an evanescent wave formed by total internal reflection of the light beam at the base of the prism.