The present invention relates to a light detecting apparatus and relates particularly, but not exclusively, to a light detecting apparatus for simultaneously detecting left- and right-handed circularly polarised light.

Two molecules are said to exhibit chirality if they are non-superposable mirror images of each other while being otherwise identical. When two molecules exhibit chirality, they are called enantiomers and can be respectively designated left-handed and right-handed.

Enantiomers respond differently when interacting with other chiral systems. For example, each enantiomer will absorb a different amount of incident circularly polarised (CP) light. Circular dichroism spectroscopy is a branch of

spectroscopy concerned with measuring this differential absorption phenomenon and using it to investigate the

enantiomeric content of a sample, which can change with time.

One approach to analysing a sample containing chiral material involves optically exciting the sample and monitoring the consequent spectrum of CP light emitted. The spectrum of CP light emitted by the sample contains information directly related to the enantiomeric content of that sample on the basis that a left-handed enantiomer emits a different spectrum of light to its right-handed counterpart. This forms the basis of circularly polarised luminescence (CPL) spectroscopy.

An existing method for monitoring the enantiomeric content of a sample includes the use of a photoelastic modulator (PEM) wherein a piece of material, such as silica, is used as an optical element driven at resonance by an alternating voltage across a piezoelectric transducer attached to the material. As a consequence of this mechanical

excitation, the material's birefringence oscillates with a frequency proportional to the mechanical resonance frequency of the optical element. The PEM then acts in a similar way to a wave plate, converting elliptically polarised light emitted by the sample into linearly polarised light. Using a PEM in a spectroscopic apparatus necessitates the use of a lock-in amplifier (LIA) to adjust the magnitude of birefringence of the PEM to appropriately convert the emitted elliptically polarised light to linearly polarised light.

A PEM is an optical element that functions as a quarter wave plate and oscillates between states of left hand and right hand polarisation of the incident light. This is subsequently converted into linearly polarised light (parallel and perpendicular) before detection.

A PEM therefore has a characteristic sampling rate limited by that frequency, which subsequently limits the rate at which data can be acquired. In addition, spectroscopic apparatus utilising a PEM cannot monitor the LCP and RCP emission spectra simultaneously and therefore cannot

independently monitor how the amounts of each enantiomer present in a sample change with time. Furthermore, the use of a PEM requires use of a LIA to modulate the mechanical excitation of the PEM, which increases the size and cost of the apparatus.

Preferred embodiments of the present invention seek to overcome one or more of the above disadvantages associated with the prior art.

According to a first aspect of the present invention, there is provided a light detecting apparatus comprising: first converting means for converting elliptically polarised light of a first type emitted by an object to linearly

polarised light of a first type; second converting means for converting elliptically polarised light of a second type emitted by said object to linearly polarised light of a second type; first detector means for providing an output signal dependent on an intensity of said linearly polarised light of said first type; and second detector means for providing an output signal dependent on an intensity of said linearly polarised light of said second type.

By providing first converting means for converting elliptically polarised light of a first type emitted by an object to linearly polarised light of a first type, second converting means for converting elliptically polarised light of a second type emitted by said object to linearly polarised light of a second type, first detector means for providing an output signal dependent on an intensity of said linearly polarised light of said first type, and second detector means for providing an output signal dependent on an intensity of said linearly polarised light of said second type, this provides the advantage of measuring both the elliptically polarised light of said first type and the elliptically polarised light of said second type independently, thereby enabling simultaneous detection of said first and second types of elliptically polarised light. Furthermore, this provides the advantage of reducing the size and cost of the apparatus.

Said first converting means and/or said second converting means may comprise at least one wave plate.

The apparatus may further comprise separating means for separating said linearly polarised light of said first type from said linearly polarised light of said second type.

The separating means may comprise at least one polarising beam splitter.

The apparatus may further comprise excitation means for causing said object to emit elliptically polarised light of said first and second types.

The excitation means may comprise at least one light- emitting diode (LED) . The excitation means may comprise at least one laser and at least one bulb containing xenon.

The excitation means may be adapted to be pulsed.

According to a second aspect of the present invention, there is provided a light detecting method, comprising: converting elliptically polarised light of a first type emitted by an object into linearly polarised light of a first type; converting elliptically polarised light of a second type emitted by said object into linearly polarised light of a second type; providing an output signal dependent on an intensity of said linearly polarised light of said first type; and providing an output signal dependent on an intensity of said linearly polarised light of said second type. The method may further comprise separating said linearly polarised light of said first type from said linearly

polarised light of said second type.

The method may further comprise causing said object to emit said elliptically polarised light of said first and second types.

The method may further comprise pulsing excitation means for causing said object to emit said elliptically polarised light of said first and second types.

A preferred embodiment of the present invention will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings, in which:

Figure 1 shows a schematic view of a light detecting apparatus embodying the present invention; and

Figure 2 shows two circular polarisation spectra as recorded by the apparatus of Figure 1.

Referring to Figure 1, a light detecting apparatus 1 embodying the present invention is shown wherein light emitted by an object in the form of a sample 2 is incident on a converting means in the form of a wave plate 4. This emitted light comprises a mixture of left-handed elliptically polarised light 6 and right-handed elliptically polarised light 8 in an unknown proportion. The characteristics of the wave plate 4 are chosen such that the elliptically polarised components 6, 8 of the light emitted by the sample 2 are converted into a first linearly polarised component 10 and a second linearly polarised component 12, polarised in a plane orthogonal to that of the first component 10 as a result of passing through the wave plate 4. These linearly polarised components 10, 12 are incident on a polarising beamsplitter 14, which separates the first component 10 from the second component 12. The first component 10 is detected by first detecting means such as a photomultiplier and/or photodiode 16, while the second component 12 is detected by second detecting means such as a photomultiplier and/or photodiode 18. The first 16 and second detecting means 18 are shown connected via respective CCD detectors 22, 24 to a processor such as a PC 20 for recording and analysis of input data. The sample 2 is illuminated by light emitted by excitation means in the form of a xenon bulb 30 energised by a pulsed pump laser 32 having passed through a monochromator 34. Referring to Figure 2, data dependent on the outputs of both first 16 and second detecting means 18 are shown on a graph displaying the intensity of each of the elliptically polarised components 6, 8 as a function of wavelength. This graph shows a significant difference between the intensity of the left-handed component 6 and the right-handed component 8 at an approximate wavelength of 610nm, demonstrating a difference in enantiomeric content of the sample 2

proportional to that difference.

It will be appreciated by persons skilled in the art that the above embodiments have been described by way of example only and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of the invention as defined by the appended claims .·