Abstract:
An ophthalmic illuminator is provided that includes a plurality of color sources, each color source producing a light of a corresponding color; a combiner for combining the light from the color sources to produced a combined light; at least one optical fiber configured to receive the combined light and propagate the received combined light towards a distal end of the ophthalmic illuminator; and a controller configured to control an intensity for each of the color sources responsive to a sampling of a spectral content for the combined light.

Description:
This application claims priority to U.S. Provisional Patent Application Ser. No. 61/349,287 filed on May 28, 2010. 
    
    
     TECHNICAL FIELD 
     This application relates to illumination in ophthalmic procedures and more particularly to ophthalmic illumination with spectral adjustment. 
     BACKGROUND 
     Ophthalmic illuminators allow a surgeon to illuminate the interior structure of the eye such as the vitreous and the retina during surgical procedures. For example, an endoscopic ophthalmic illuminator (endo-illuminator) includes an optical fiber within the bore of a cannula. By driving a proximal end of the optical fiber with a suitable light source, light emitted from a distal end of the fiber illuminates the desired portion of the eye during a surgical procedure. Alternatively, a physician may illuminate the eye with fiber optic illumination while using an ophthalmic microscope. 
     One factor for ophthalmic illumination is the spectral output. In general, biological tissue is a broadband reflector such that white light illumination is preferable. However, there are situations such as the use of dyes or the detection of certain proteins in which a physician will prefer a suitably colored illumination. In general, most conventional white light sources such as a white light LED provide a fixed spectral output. However, the desired spectral output for ophthalmic illumination may vary depending upon numerous factors that are difficult to predict a priori such as: the physical appearance of the retina; the desired illuminance (luminous flux/area) in the operating field, the desired Color Rending Index (CRI) for adequate visualization of the retinal tissue; and the degree of contrast enhancement or color suppression desired during the surgery; Another factor for ophthalmic illumination is the resulting aphakic hazard potential, which is the potential for light-induced photochemical damage to the retina. In general, there is virtually no aphakic hazard at wavelengths longer than 510 nm. The aphakic hazard can thus be virtually eliminated by using only those wavelengths longer than 510 nm. Thus, the desired spectral output for ophthalmic illumination may also vary according to the expected length of the operation and the proximity of the distal end of the illumination probe to the retina. 
     Accordingly, there is a need in the art for an improved ophthalmic illuminator that adjusts its output spectrum in real time. 
     SUMMARY 
     In accordance with a first aspect of the disclosure, an ophthalmic illuminator is provided that includes: a plurality of color sources, each color source producing a light of a corresponding color; a combiner for combining the light from the color sources to produced a combined light; at least one optical fiber configured to receive the combined light and propagate the received combined light towards a distal end of the ophthalmic illuminator; and a controller for controlling an intensity for each of the color sources responsive to a sampling of a spectral content for the combined light. 
     In accordance with a second aspect of the invention, a method of controlling an ophthalmic illumination is provided that includes: producing a plurality of lights of different colors; combining the plurality of lights of different colors to produce a combined light; illuminating an operating field within a human eye with the combined light; measuring a spectral radiometric function for the combined light during the illumination; and responsive to the measurement, adjusting an intensity for at least one of the lights of different colors. 
     In accordance with a third aspect of the invention, an ophthalmic illuminator is provided that includes: a red LED; a green LED; a blue LED; a combiner for combining light from the LEDs to produce a combined light; at least one optical fiber configured to receive the combined light and propagate the received combined light towards a distal end of the ophthalmic illuminator; and a controller configured to control an intensity for each of LEDs responsive to a sampling of a spectral content for the combined light. 
    
    
     
       DESCRIPTION OF FIGURES 
         FIG. 1  illustrates a spectrally-adjustable ophthalmic fiber illumination probe. 
         FIG. 2  is a graph of the aphakic hazard function. 
         FIG. 3  illustrates the spectral power distribution bands for various color LED sources. 
         FIG. 4  is a graph of the CIE tristimulus functions. 
     
    
    
     Embodiments of the present invention and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures. 
     DETAILED DESCRIPTION 
     To provide spectral output selectivity and brightness flexibility, a plurality of color light sources are selectively combined to produce an illuminating light having a desired spectral output. In this fashion, a clinician may thus not only alter the spectral output for the illumination but also vary (or keep constant) the resulting luminous flux despite the spectral augmentation. Turning now to the drawings,  FIG. 1  illustrates a spectrally-adjustable ophthalmic illuminator probe  100 . Illuminator  100  includes a red LED source  105 , a blue LED source  110  and a green LED source  115 . Each LED associates with a corresponding collimating lens. Thus, red light from red (R) LED source  105  is collimated through a lens  120 , blue light from blue (B) LED source  110  is collimated through a lens  125 , and green light from green (G) LED source  115  is collimated through a lens  130 . The resulting collimated light beams are received at a combiner  135  to produce a combined light beam  140 . Combiner  135  may comprise a Phillips prism, a dichroic cube, or other suitable optical combiner. 
     Because of the RGB contribution from the sources, combined light beam  140  may nominally be a white light beam. To provide spectral variability in addition to white light illumination, the radiant flux (intensity) for each light source may be tuned as desired. For example, a variable current amplifier  145  may vary the radiant flux produced by red LED source  105 , a variable current amplifier  150  may vary the radiant flux from blue LED source  110 , and a variable current amplifier  155  may vary the radian flux for green LED source  115 . Other techniques may also be used to vary or tune the light intensity from each source. For example, constant power sources may drive the LEDs through pulse width modulators. A controller  160  such as a microprocessor or microcontroller controls the radiant flux from each light source accordingly. Thus, for illuminator  100 , controller  160  controls the amount of gain applied by each current amplifier. 
     In one embodiment, controller  160  automatically adjusts the gain responsive to feedback as sensed through an optical sampler  165  that samples combined beam  140 . For example, optical sampler  165  may comprise a beam splitter or a folding mirror to split off a relatively small portion of the combined beam  140  as a sampled beam  170 . To analyze the spectral content of the sampled beam  170  (and thus of combined beam  140 ), controller receives data from a spectroradiometer  190  that receives sampled beam  170 . 
     The remaining unsampled portion of combined beam  140  is received by a condensing optic lens  180  so as to couple into an optic fiber  185  (or optic fiber bundle). Fiber  185  may thus be the illumination source in ophthalmic instruments such as an ophthalmic microscope, slit lamps, indirect ophthalmoscopes, and fiber endo-illuminators. Controller  160  is also responsive to user input such that a physician may manually command the appropriate gains so as to achieve the desired spectral content for combined beam  140 . 
     In that regard, the general desire for white vitreoretinal illumination flows from the phenomenon of color rendering, which is the ability of the illuminating light to render the appearance of various colors as they should appear to the human observer. To help indicate how colors will appear under spectrally-different light sources, a color rendering index (CRI) has been derived as known in the optic arts. In general, the more spectrally broadband a source is, the higher its CRI value will be. White light illumination thus has a high CRI value. But as the illumination takes on color, the CRI index will drop. For example, illumination at wavelengths only of 510 nm in wavelength or longer will have a relatively low CRI. As will be discussed further herein, controller  160  may automatically control the CRI by adjusting the radiant flux from each source to achieve a desired chromaticity value as defined by, for example, the International Commission on Illumination (CIE) 1931 color space. 
     The automatic control provided by controller  160  in response to sensing the spectral content of the combined light advantageously minimizes or eliminates aphakic hazard in certain embodiments. In that regard, stringent guidelines on total aphakic exposure such as 10 J/cm 2  may be satisfied, thereby increasing patient safety. For controller  160  to properly calculate the total aphakic irradiance, a correlation between the irradiance at the retina and the radiant flux measured by spectroradiometer  190  of sampled beam  170  is useful. The irradiance on the retina depends upon a number of factors such as the separation between an emitting distal end of a probe holding fiber  185  and the retina. As the distal end of the probe moves closer to the retina, the more intense will be the retinal irradiance will increase. A typical separation between the probe and the retina for conventional endo-illuminators is 5 to 15 mm. However, for a laser probe such as used in retinal photocoagulation therapy, the separation may be in the range of 2 to 4 mm. Another factor affecting time-averaged retinal irradiance is whether the illumination probe is pointed at the same area of the retina as opposed to sequentially moving the illumination to different portions of the retina. Other factors include the spread angle of the emitted light beam, the incident angle for the emitted light beam onto the retina, and the detailed structure and physical condition of the retina as well as obscuring effects of other tissues such as vitreous and epiretinal membranes. 
     To assist controller  160  in making an a priori estimate of the expected irradiance, illumination probe  100  may include an RFID tag  195  so that an RFID interrogator (not illustrated) may read associated RFID data from tag  195  and provide the data to controller  160 . For example, an estimate of expected conditions and beam spread angle associated with a given probe is loaded onto tag  195  so that controller  160  can correlate between spectral power measurements of sampled beam  170  and a corresponding irradiance at the retina. 
     Spectroradiometer  190  may sample the entire visible spectrum for sampled beam  170  or merely sample the spectral power at selected wavelengths having the expected predominant optical energy. Having determined some suitable radiometric measure (denoted as R) such as radiometric flux or irradiance at the sampled wavelengths, controller  160  may thus construct a corresponding spectral radiometric function R′(λ). e.g. watts/nm. An accurate measure of the aphakic hazard requires a translation of such a radiometric quantity to an aphakic radiometric quantity such as aphakic irradiance or aphakic radiometric power. To calculate an aphakic radiometric quantity, controller  160  retrieves the aphakic hazard function A(λ) as illustrated in  FIG. 2  from a memory  161 . Controller  160  then numerically integrates according to the following equation:
 
 R   aph   =∫R′ (λ) A (λ) dλ 
 
where R aph  is an aphakically weighted radiometric quantity as determined by the type of radiometric quantity (radiometric flux, irradiance, etc.) used to establish R′(λ). Controller  160  may then determine the total aphakic exposure over the procedure time by multiplying R aph  by the retinal illumination duration. Should controller  160  determine that the aphakic exposure has exceeded some maximum threshold such as 10 J/cm 2 , controller  160  may then reduce or eliminate the emission from any color light source having wavelengths less than 510 nm.
 
     Another consideration besides the aphakic hazard that controller  160  may address is contrast, which is typically defined as the luminance ratio between the brightest and dimmest portions of the retinal image. Whether a particular retinal tissue reflects strongly or weakly depends on its reflectance spectrum multiplied by the illumination spectrum as integrated over the visible wavelengths. If a particular retinal tissue is highly absorptive over a spectral region corresponding to one of the color sources but a different retinal tissue is highly reflective at that same wavelength, controller  160  could increase the contrast between the two retinal tissues by suppressing the remaining color sources. Alternatively, there may be high contrast in the presence of white light (full spectrum) illumination such that controller  160  tunes the color sources to effect white light illumination. Conversely, low contrast may be achieved with single color source illumination. Depending upon the circumstances of a particular therapy, either contrast enhancement or suppression may be desirable. The degree of suppression or enhancement depends upon the spectral behavior for the various light sources and the spectral reflectance characteristics of the tissue being observed. As seen in  FIG. 3 , which illustrates the spectra for various commercially-available LED color sources, these sources typically have relatively narrow spectral bandwidths. Such narrowband emission enhances the ability to increase or suppress contrast as desired by controller  160 . 
     To achieve a particular chromaticity, controller  160  may be configured to measure the chromaticity coordinates for the current illumination. In that regard, memory  161  may store CIE tristimulus functions  z (λ),  y (λ), and  x (λ) as illustrated in  FIG. 4 . Controller  160  may then retrieve these functions and determine the corresponding CIE primaries  X ,  Y , and  Z  by numerically integrating the following equations:
 
   X =∫R′ (λ)    x   (λ) dλ 
 
   Y =∫R′ (λ)    y   (λ) dλ 
 
   Z =∫R′ (λ)    z   (λ) dλ 
 
where R′( ) is the spectral radiometric function discussed above. Given the CIE primaries, controller  160  may then calculate the x and y CIE chromaticity coordinates according to the following equations:
 
 x=  X   /(   X +  Y +  Z   )
 
 y=  Y   /(   X +  Y +  Z   )
 
Controller  160  may thus monitor the chromaticity values and tune the various color source intensities accordingly to achieve a desired effect. For example, controller  160  may adjust color rendering or contrast in this fashion.
 
     Although illuminator  100  has been discussed with regard to three independent color sources, it will be appreciated that white light illumination can be achieved with just two sources. Conversely, rather than just use a RGB combination as discussed above, a greater number of color channels may used such as four, five, or more color channels may be implemented. In addition, the spectral content of the combined light may be characterized using a color camera instead of a spectroradiometer. 
     Embodiments described above illustrate but do not limit the invention. It should also be understood that numerous modifications and variations are possible in accordance with the principles of the present invention. Accordingly, the scope of the invention is defined only by the following claims.