Abstract:
A teaching aid for demonstrating the component pure spectral colors of white light and the effect of selective recombination thereof. It utilizes the diffraction principle to produce a spectrum from white light and selectively filters the spectrum. By splitting the filtered light prior to recombination, it is possible to project an image of the selective filter and the recombined selectively filtered light adjacent to each other on a projection screen. Thus, the effects of the variation of the filter is shown directly by a variation in the color and intensity of the recombined light.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a teaching aid which demonstrates the component pure spectral colours of white light and the effect of selective recombination of a particular group and intensity of those pure spectral colours to form an area of light of uniform colour. 
     2. Description of the Prior Art 
     Historically, Sir Isaac Newton was one of the first persons to conduct an experiment whereby the colours of the spectrum produced by passing white light through a first prism were recombined by means of a second prism or lens to produce the original white light, thereby demonstrating conclusively that white light was a mixture of spectral colours. Since then, the sciences of colourimetry and spectrophotometry have become extremely complex yet many of the principles and equipment used today are based on the conclusion originally demonstrated by Newton. This invention relates to an aid for the teaching and understanding of these basic principles. 
     SUMMARY OF THE INVENTION 
     To this end, in one of its aspects, the invention provides an improved teaching aid to demonstrate the component pure spectral colours of white light and the effect of selective recombination of a particular group and intensity of these pure spectral colours to form an area of light of uniform colour. 
     The invention provides a teaching aid for demonstrating the principle of selective colour combination and recombination which comprises a light source emitting light, a diffraction means adapted to diffract said emitted light, a selective spatial filter means adapted to selectively filter the diffracted light, the spatial filter being placed at the spectrum formed by the diffracted light, a first focusing means placed subsequent to and near the selective spatial filter means adapted to focus the selectively filtered light to a subsequent first viewing position, a beam splitting means adapted to split the focused and filtered light into at least one split beam and one transmitted beam, the beam splitting means placed subsequent to the first focusing means and in front of the subsequent first viewing position, focusing means adapted to focus the split beam to a subsequent second viewing position. 
     In another of its aspects, the invention provides a teaching aid which comprises a light source illuminating a vertical slit and a first focusing means adapted to focus the light emitted from the slit to a blazed diffraction grating, a selective spatial filter placed in the spectrum of the light emitted from the grating, a second focusing means placed subsequent to and near the selective spatial filter, the second focusing means adapted to focus the selectively filtered light to a subsequent first viewing position, a beam splitting means adapted to split the focused light into a split beam and a transmitted beam, a relay lens adapted to relay the split beam to a mirror, the mirror adapted to reflect the split beam to a subsequent second viewing position, the subsequent second viewing position of the split beam and the subsequent first viewing position of the selectively filtered transmitted beam being in the same conjugate plane. 
     Further objects and advantages of the invention will appear from the following description taken together with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of the invention of the present invention. 
     FIG. 2 is an isometric view of the invention of the present invention. 
     FIG. 3 illustrates a simple diffraction grating. 
     FIGS. 4 to 7 illustrate various selective filters used in the present invention. (These figures appear on the page with FIG. 1.) 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention discloses a device which demonstrates the principle of additive and subtractive colour. The word &#34;colour&#34; in the present specification is used in the visual sense to describe the integrated effect of many pure spectral colours rather than to the pure spectral colours themselves. 
     The principles of additive and subtractive colour in relation to a three colour system may be clearly demonstrated by the following device. 
     Referring to FIG. 1, a light source 10 emits white light which impinges on a dispersion means 12. The white light is then dispersed according to its wavelength and passes through a focusing lens 14 and is imaged at position 16. A selective spatial filter means 18 is placed before the lens 14 and this filter selectively filters the light from the dispersion means 12 onto the focusing lens 14. A beam splitter 20 is placed subsequent to the lens 14 and in front of the position 16. The beam splitter 20 splits the filtered light 22 and reflects a portion thereof to a relay lens 24 which in turn relays the split beam 26 to a reflecting device 28 which in turn reflects the light to a screen 30. 
     The screen 30 thus becomes a function of the selective filter 18 and demonstrates which parts of the spectrum are selectively filtered. The viewing position 16 shows the resulting colour of the light after the filter 18 selectively filters the light. Thus, if the screen 30 and the position 16 are placed on a projection screen, the effect of selectively filtering dispersed white light is shown by the integrated result of the light at the position 16. When the design of the selective filter is varied, these variations are shown on the screen 30 as a function of the filter 18 and the effect is clearly seen by the integrated result at position 16. 
     The source 10 may comprise a standard 35 mm slide projector having a light source and a projection lens. As shown in FIG. 2, the projector 32 is used to project an image of a vertical slit 34 to a position 36. The present invention is not restricted to one utilizing this projector and it is recognized that any suitable source of white light may be substituted therefore. 
     The dispersing element 12 is placed immediately after the projection lens 38. It is understood that any dispersion means may be used and the preferred embodiment of the device utilizes a diffraction means as the dispersion element and reference will now be made to the device utilizing a diffraction means. 
     The simplest form of a diffraction grating is a set of parallel lines or grooves as seen in FIG. 3. The light is diffracted at the lines or grooves depending upon the wavelength the longer the wavelength, i.e., the red, the greater the angle of diffraction will be. 
     When a collimated or parallel beam of white light is directed through a diffraction grating and then a lens, a spectrum is produced. The grating itself may be &#34;blazed&#34; to produce preferentially the maximum light in a particular order if desired. 
     The spread or the dispersion angle of the light emerging from the diffraction grating can be calculated according to the following drawing and equation: ##STR1## wherein λ represents the wavelength of the light, 
      a represents the grating spacing, 
      m represents the order 0 ±1 ±2 etc. 
     
         Sin α + Sin β = (mλ)/a 
    
     Thus, if the incident beam is normal to the grating, then α = 0. In the first order, where m= ±1, 
     
         Sin β = λ/a 
    
     Thus, the angle β changes as Sin -1 (λ/a) or the wavelength of the light. For small angles, this is approximately in direct proportion to λ . 
     If a line or slit source of light is used and is collimated by a first lens onto the diffraction grating, the beam will be deviated by the diffraction grating dependent upon the wavelength. The beams are then brought to a focus by a second lens forming a set of slit images of the spectrum of the source in each wavelength. 
     The resolution depends upon the width of the slit. A large slit width means that each wavelength has considerable overlap and the colours are partially integrated or mixed. 
     The diffraction gratings may be both of the reflection or the transmission type and the same application is applicable. 
     For multiple orders, the following occurs: when m= 0. 
     
         Sin α + Sin β = 0 
    
     
         Sin α = Sin β 
    
     
         α = β 
    
     Thus, there is no dependence upon the wavelength and the light remains white, and is not dispersed or deviated. 
     when m= 1, 
     
         Sin α + Sin β = 1λ/a 
    
     when m= -1, 
     
         Sin α + Sin β = -1λ/a 
    
     Similarly, the same equation is applicable for all orders of m limited by Sin β = 1. 
     It has been found that blazed gratings put up to 70% of the total light into a single order (usually m= +1). 
     Thus, when a collimated beam of white light is passed through a diffraction grating followed by a lens, a spectrum is produced. 
     The selective spatial filter 18 is placed in the spectrum formed by the dispersion means 12. A pair of guide members 40, 42 may be placed in the same plane as the spectrum to hold the filter 18 in place, in order the various parts of the spectrum can be blocked. 
     The selective filter 18 may be of infinitely variable shapes. FIGS. 4, 5, 6 and 7 show four possible shapes of filters which may be used in the present device. Filter 44 shows a blocking of the green light; filter 46 shows a blocking of blue light; filter 48 shows a blocking of red light and filter 50 shows an arbitrary pattern of blocking. It is clearly seen that for demonstration and teaching purposes, an infinitely large number of spatial filters may be designed to illustrate the principle of the present invention. 
     The filtered light then impinges on the lens 14 which projects the image of the aperture of the grating 12 onto the position 16. A beam splitter 20 (preferably a 1:1 splitter with as flat spectral response as possible) is inserted subsequent to lens 14. The split beam is passed through the imaging lens 24 and by means of reflector 28, forms the image of the spectrum and the selective filter 18 on the screen 30. The image produced on the screen 30 (referred to as the first image) would be a function of the selective filter 18. 
     The light which is transmitted by the beam splitter 20 is focused at position 16. This image (referred to as the second image) is the integrated or recombined light of the spectrum transmitted at the selective filter 18. Thus, the filter 18 acts as an aperture stop for the lens 14. As the light at the aperture of the lens 38 is not dispersed, i.e., white light, so also is its image at position 16 not dispersed. 
     If no filter at position 18 is used, the image at position 16 will be white and the spectra at filter 18 and screen 30 is complete with all colours. If filters as shown in FIGS. 4 to 6 are used (green light, blue light and red light being removed respectively), then the spectra at screen 30 will appear as &#34;red-blue&#34;, &#34;red-green&#34;, and &#34;blue-green&#34; respectively. The colour at the position 16 will appear as magenta, yellow and cyan. If a filter as shown in FIG. 7 is used, which is shown predominately transmitting in the red and blue portions of the spectrum, the colour at the position 16 would appear purple. 
     Subtractive colour principles may also be demonstrated by the present device. For example, if a cyan and yellow filter were combined, green light would be produced. By removing the red from the cyan and the blue from the yellow, only the light common to both filters would be transmitted, that is, green. 
     Thus, it can be seen that the principles of colour addition and subtraction may be demonstrated by the device of the present invention. 
     The device may be used as follows to demonstrate the component parts of white light and the effects of their selective combination. 
     Filters are used which remove single colours from the spectrum. If the filter as seen in FIG. 4 were used, then only the red and the blue light would be transmitted and the colour at the viewing position 16 would appear magenta. At position 30, the image of the filter would appear with a red band on one side and a blue band on the opposite side with the green band removed. Thus, the student would see first at position 30, these colours which were transmitted and the colours which were filtered out and then the effect of this filtering would be seen by the colour exhibited at position 16. 
     By using the filters as shown in FIGS. 4, 5 and 6, the principles of colour addition and subtraction can be clearly shown. After demonstrating the aforenoted principle, the principles of spectral distribution can be demonstrated by using filters such as the one illustrated in FIG. 7. These filters do not necessarily block out an entire wavelength and transmit other wavelengths but they filter out various portions of the selected wavelengths. Thus, the intensity of each wavelength passing through the filter may be controlled. Again, the effect of varying the intensities of the transmitted wavelengths will be shown by changing the colour at the position 16. 
     Although the disclosure describes and illustrates a preferred embodiment of the invention, it is to be understood that the invention is not restricted to this particular embodiment.