Cooke triplet
Encyclopedia
The Cooke triplet is a photographic lens
designed and patent
ed (patent number GB 22,607) in 1893 by Dennis Taylor
who was employed as chief engineer by T. Cooke & Sons of York. It was the first lens system that allowed elimination of most of the optical distortion or aberration
at the outer edge of lenses.
A Cooke triplet comprises a negative flint glass
element in the centre with a crown glass
element on each side. In this design, the sum of all the curvatures times indices of refraction can be zero, so that the field of focus is flat (zero Petzval field curvature
). In other words, the negative lens can be as strong as the outer two combined, when one measures in dioptres, yet the lens will converge light, because the rays strike the middle element close to the optic axis. The curvature of field is determined by the sum of the dioptres, but the focal length is not.
At the time, the Cooke triplet was a major advancement in lens design. It was superseded by later designs in high-end camera
s, but is still widely used in inexpensive cameras, including variations using aspheric elements
, particularly in cell-phone camera
s.
This parallels the fact that John Dollond had in turn patented the achromatic doublet in the 1750s, though it had been used decades earlier.
Binoculars
as well as refracting telescopes often use triplets. The same holds for many projection
lenses, e.g., for 35 mm slide projector
s.
A similar design is used in the strong focusing synchrotron
, invented first by Nicholas Christofilos
in 1949, but his work was not known in the U.S., where parallel development took place.
Photographic lens
A camera lens is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capable of storing an image chemically or electronically.While in principle a simple convex lens will suffice, in...
designed and patent
Patent
A patent is a form of intellectual property. It consists of a set of exclusive rights granted by a sovereign state to an inventor or their assignee for a limited period of time in exchange for the public disclosure of an invention....
ed (patent number GB 22,607) in 1893 by Dennis Taylor
Harold Dennis Taylor
H. Dennis Taylor FRS was a British optical designer and inventor, chiefly famous for the invention of the Cooke Triplet, although he was granted about 50 other patents....
who was employed as chief engineer by T. Cooke & Sons of York. It was the first lens system that allowed elimination of most of the optical distortion or aberration
Aberration in optical systems
Aberrations are departures of the performance of an optical system from the predictions of paraxial optics. Aberration leads to blurring of the image produced by an image-forming optical system. It occurs when light from one point of an object after transmission through the system does not converge...
at the outer edge of lenses.
A Cooke triplet comprises a negative flint glass
Flint glass
Flint glass is optical glass that has relatively high refractive index and low Abbe number. Flint glasses are arbitrarily defined as having an Abbe number of 50 to 55 or less. The currently known flint glasses have refractive indices ranging between 1.45 and 2.00...
element in the centre with a crown glass
Crown glass (optics)
Crown glass is type of optical glass used in lenses and other optical components. It has relatively low refractive index and low dispersion...
element on each side. In this design, the sum of all the curvatures times indices of refraction can be zero, so that the field of focus is flat (zero Petzval field curvature
Petzval field curvature
Petzval field curvature, named for Joseph Petzval, describes the optical aberration in which a flat object normal to the optical axis cannot be brought into focus on a flat image plane....
). In other words, the negative lens can be as strong as the outer two combined, when one measures in dioptres, yet the lens will converge light, because the rays strike the middle element close to the optic axis. The curvature of field is determined by the sum of the dioptres, but the focal length is not.
At the time, the Cooke triplet was a major advancement in lens design. It was superseded by later designs in high-end camera
Camera
A camera is a device that records and stores images. These images may be still photographs or moving images such as videos or movies. The term camera comes from the camera obscura , an early mechanism for projecting images...
s, but is still widely used in inexpensive cameras, including variations using aspheric elements
Aspheric lens
An aspheric lens or asphere is a lens whose surface profiles are not portions of a sphere or cylinder. In photography, a lens assembly that includes an aspheric element is often called an aspherical lens....
, particularly in cell-phone camera
Camera phone
A camera phone is a mobile phone which is able to capture still photographs . Since early in the 21st century the majority of mobile phones in use are camera phones....
s.
Early designs
Despite the fact that the Cooke design was patented in 1893 it seems that the use of achromatic triplet designs in astronomy appeared as early as 1765. The 1911 Encyclopædia Britannica wrote:- The triple object-glass, consisting of a combination of two convex lenses of crown glass with a concave flint lens between them, was introduced in 1765 by PeterPeter DollondPeter Dollond was an English maker of optical instruments, the son of John Dollond. He is known for his successful optics business, and for the invention of the apochromat.-Biography:...
, son of, John DollondJohn DollondJohn Dollond was an English optician, known for his successful optics business and his patenting and commercialization of achromatic doublets.-Biography:...
, and many excellent telescopes of this kind were made by him.
This parallels the fact that John Dollond had in turn patented the achromatic doublet in the 1750s, though it had been used decades earlier.
Application
The triplet soon became a standard in lens design still used with low-end cameras today. The main optical manufacturers often further developed the original Cooke triplet (e.g., the Zeiss Triotar) that were produced for many decades.Binoculars
Binoculars
Binoculars, field glasses or binocular telescopes are a pair of identical or mirror-symmetrical telescopes mounted side-by-side and aligned to point accurately in the same direction, allowing the viewer to use both eyes when viewing distant objects...
as well as refracting telescopes often use triplets. The same holds for many projection
Slide projector
A slide projector is an opto-mechanical device to view photographic slides. Slide projectors were common in the 1950s to the 1970s as a form of entertainment; family members and friends would gather to view slide shows...
lenses, e.g., for 35 mm slide projector
Slide projector
A slide projector is an opto-mechanical device to view photographic slides. Slide projectors were common in the 1950s to the 1970s as a form of entertainment; family members and friends would gather to view slide shows...
s.
A similar design is used in the strong focusing synchrotron
Synchrotron
A synchrotron is a particular type of cyclic particle accelerator in which the magnetic field and the electric field are carefully synchronised with the travelling particle beam. The proton synchrotron was originally conceived by Sir Marcus Oliphant...
, invented first by Nicholas Christofilos
Nicholas Christofilos
Nicholas Constantine Christofilos was a Greek-American physicist.Christofilos was born in Boston, Massachusetts and raised in Greece...
in 1949, but his work was not known in the U.S., where parallel development took place.