Machine Vision Lens and Camera Selection Fundamentals (Part 1)

It has been a brief respite since I last written. Taking on a different hue, we discuss the essentials on lens and camera selection for an imaging project.

For any machine vision project, the very first thing to do is to determine the lens and the resolution of the camera used. In order to do so, it requires the knowledge on the smallest resolvable resolution and the operating field of view (FOV) needed by the application. In most cases these details are application and imaging algorithm dependent (this will be discussed on a separate article).

In this illustration, the type of lens used is assume to be CCTV lenses. 

Given a vision algorithm that requires 1 single pixel to be represented as 0.1mm and the field of view for the application has to be 10mm x 10mm, a camera of at least 1000 x 1000 pixels (10mm divided by 0.1mm) will be needed. 

Select a camera with pixel size of 5.5um (assuming kodak sensor) and 1000 x 1000 pixels, the lens will need to project the required image onto the pixels of the image sensor within the area of 5.5mm x 5.5mm.

Knowing the FOV (Field of View) and the projected image size on the sensor, the lens can be estimated.

Using thin lens assumption, the magnification of the image is almost in ratio of the focal length f, and the working distance. Selecting a 50mm (f) lens will give the following working distance

Working distance (WD) = (H*f)/H’ + f = (10/5.5) x 50 + 50 = 141mm

In the earlier result the back focal distance is not included in the calculation. For a C mount lens, the back focus distance (fb) is about 17.52mm. To maintain a focus image, the projected image on the image sensor is constrain by the following thin lens formula

1/f = 1/(fb+f) +1/(WD)  Eq2.0

Applying f = 50mm and fb = 17.52mm, the equation gives a working distance of 192.8mm.

With a working distance of 192.8mm the needed magnification or FOV cannot be realized (see Eq1.1), the working distance have to further reduce in order to increase the magnification. To do that, fb has to be increased by adding extension rings.

Apply f = 50mm and WD = 141mm

Using thin lens formular again, fb = 27.47mm

The total extension distance needed = 27.47-17.52mm ≈ 10 mm.

Following summarize the results in this example,

•  FOV 10mm (given)
• Per pixel representing 0.1mm (given)
• Camera, 5.5um pixel size with 1k x 1k resolution (esitmate from FOV and resolution per pixel)
• Lens C mount 50mm ( make a selection )
• Working distance 141mm
• Extension ring 10mm

This illustration exhibits a quick way to estimate the required lens setup and camera without having looking up at lens specification. A more accurate means of lens determination will require the use of the view angle instead of focal length provided by the lens specification. (to be discuss on a separate article)

A new age for digital interaces, Coaxpress and Camera Link HS

Coaxpress and Camera Link HS emerged after the unsuccessful attempt on VISILink. The event unfolds by these two digital interfaces in industrial cameras draw parallels to the iconic video format war in the consumer market. In the 70s we have VHS verse Betamax and in the recent millennium, HD-DVD verses Blue-Ray. If there is anything to learn from history, neither the best nor the cheapest technology will emerge as the victor.

Of the most lucrative and largest volume in the high bandwidth industrial digital camera is the sales of linescan cameras. In linescan applications there is endless request for higher line speed and larger data bandwidth, this poses a challenge to the existing bandwidth of CameraLink standard as sensor technology advanced. The demand for a new high speed digital interface arises.

Dalsa with its linescan solution has precipitated a formidable wall in the industry. Dalsa not only actively seek for available technology but also invested heavily on prototypes. With opportunity arises during the flat panel and LEDs era in the mid 2000s, Dalsa announced its solution to serve the market request, HS-Link together with Piranha HS 12K TDI camera in 2009. Thereafter, they take this development to AIA to adopt it as Camera Link HS.

Coaxpress is originally developed by five companies and anointed by JIIA. With many anxious to jump onto the high speed wagon, Coaxpress is now being supported by many cameras and framegrabber card vendors. Among them, e2v and Active Silicon presented an attractive alternative to Dalsa Camera Link HS 16k linescan solution (ELiiA 16k linescan camera with FireBird framegrabber card). Compared to Dalsa monochromic domination in Camera Link HS, the Coaxpress technology appears to fill with genetic diversity and vibrancy.

From the technological perspective, Coaxpress uses EqcoLogic transceiver while Camera Link HS leverages on 8b/10b Gigabit Ethernet technology. 

		Camera Link HS	CoaXpress	Camera Link
Bandwidth		6000MB/s	12000MB/s	850MB/s
Trigger		3.2ns Jitter	12ns Jitter	0 ns Jitter
Distance		15m on CX4	100m	10m
Com Channel		300MB/s	2.1MB/s	1MB/s
Integrated into FPGA		Yes	No*	Yes

*Today there is an untested alternative from kaya instrument

With bandwidth double of Camera Link HS and superior running distance, at a single glance, CoaXpress looks set to replace Camera Link HS. However, the winner of the consumer video format war has taught us that superior technological and lower price does not guarantee success. Camera Link HS have some very distinctive advantages, Dalsa has been dominating the high end linescan (TDI) market for years, there is already a very well established integrated Dalsa camera and framegrabber solution and by exploiting network technology (8b/10b) there is a possibility of cheaper solution in the near future by third party framegrabber cards.  There is third party framegrabber card vendor pledging support on Camera Link HS under the IAI flag and evidence of any serious challenges against Dalsa linescan domination at this moment is still exiguous.         

The die has been cast and the clash between the two standards in the linescan arena is inevitable. The cameras technology available from either side will act as the hammer whiles the interface standard as the anvil. A new chapter for linescan inspection will be forged with higher line speed and higher sensor resolution. 

USB3 Vision: A force not to be reckoned with

The birth of modern vision industry happened during the 90s computer boom . With personal computer becoming cheaper and faster during that era, running machine vision algorithm no longer requires expensive investment in hardware development such as DSP and FPGA. Back then machine vision related companies were mushrooming in every corner of the developed world.

The digital evolution of industrial camera is fuelled by the ever growing appetite for higher resolution and faster frame rate. The machine vision industry looks towards digital interface when traditional analogue standards can no longer coped with the increasing demand.

During that period, with no single standard digital format, interoperability was near impossible. In October 2000, the very first digital standard known as Camera Link was introduced. It was, and still is, an expensive technology, but semiconductor equipment makers embraced it quickly as the need for quality inspection technologies exceed the cost it incurred. Unnoticeable by some, the Camera Link standard has actually fired off the first servo in the race for dominance in the digital age of modern machine vision .

Within a year after the induction of Camera Link, Pointgrey and AVT pioneered the firewire standard (1394A) developing range of successful low cost digital format cameras for the industry. The salient of this new technological front are AVT’s Dolphin series and Pointgrey’s Firefly series. Almost immediately, these cameras enter the industry like a welcoming storm from a long drought, shaking up the mainstream and low end machine market. Applications that require higher resolution and yet at a lower speed now have an alternative solution, Firewire instead of Camera Link. With the success of Firewire, industry innovators are now in two camps. With one camp tirelessly looking into high end dedicated imaging interfaces (deriving today HS-link and Coaxpress standard), predicting that this is the future industry needs.  While the other believes that there should be a low cost solution and ploughs ahead with commercial standards.

In 2006, the GigE Vision standard was inaugurated; it provided a much larger bandwidth and at the same time eliminated the limitation of cabling distance of firewire technology. Among the early adopters of this technology was Basler. The Basler Pilot series with Kodak sensor together with their Scout series with Sony sensor was first introduced in 2006. The impact of GigE Vision became apparent when Basler launches her Ace product near the end of 2009. Its success lies not only in her indigenous design but also the strategic alliances with major industry partners such as National Instruments and Cognex .

Today, USB3 Vision emerged as the new mainstream digital standard. Unlike its predecessors (GigE Vision and Firewire), USB3 Vision is more than a replacement technology for the earlier mainstream market. It has the great potential to penetrate into the untouchable high end area scan market that had been continuously dominated by Camera Link technology for the last 10 years.

Camera Link Base interfaces with a bandwidth of 255Mbyte/sec makes up the major demand for area scan Camera Link market. USB3 Vision with its 400Mbyte/sec bandwidth provides a competitive alternative to Camera Link Base. There is one major drawback in earlier technology such as GigE or Firewire, CPU loading for camera acquisition gets significant with increasing amount of data steamed to PC. USB3 is different from such earlier consumer technology, thanks to the use of DMA (Direct Memory Access), CPU loading becomes negligible.

	Camera Link Base	Firewire	GigE Vision	USB3 Vision
Bandwidth	255MB/s	80MB/s	100MB/s	400MB/s
CPU Loading	Low	High	High	low
Triggering Concurrency on multiple cameras	High	Low*	Low	Low

Most OEM equipment makers are now keeping a keen eye on this technology and are seeking for the perfect moment to adopt the change. In 2 to 3 years time, the proliferation of USB 3.0 hardware on PC will become prevalence and in tandem, development in the consumers industry is working towards an overwhelming bandwidth of 10GB/s with USB 3.5. Already a Taiwanese chip maker, ASMedia, has planned to release her first USB 3.5 host controller by 2014. There are definitely plenty of rooms for improvement in the USB vision technology. A whole new world will emerge for machine vision as the evolution of USB technology in the consumer industry continues to advance. It is indeed an exciting time for the future of the industry.

IMX174, Sony's ground-breaking global shutter CMOS sensor for machine vision

Without a shadow of a doubt, Sony is among the big names in the imaging sensor industry. In the dusk of 2013, it releases its first global shutter CMOS sensor IMX174. With its Exmor CMOS technology, it possess unprecedented dynamic range of more than 70dB as a CMOS sensor. Fix pattern noise that is commonly found on CMOS is basically non existence. The read noise is exceptionally low with only ~6 electrons. Coupled with its large pixel size of 5.68um, this sensor is extremely sensitive to light and it deliver up to a maximum of ~160fps at 1920 x 1200 resolutions.

In the past, KAI series sensor based cameras from Truesense have always been the obvious choice for most OEM equipment makers that have requirement less than 4 Mpx resolution in high speed (~50 to >100fps) defects inspection.  The wind of change is blowing, with the prevailing Exmor technology from IMX174 even the most recent KAI-2170 Truesense sensor is being eclipsed by this Sony technology .

	KAI-2170	IMX174
Linear Dynamic Range (est)	~70dB	~70dB
Read Noise	12 e	6.83 e
Full Well	44ke	32.5ke
Pixel Size	7.4 x 7.4 um	5.68 x 5.68um
Maximum Quantum efficiency	52% @ 500nm	76% @525nm
Resolutions	1920 x 1080	1920 x 1200
Maximum frame rate	Quad Output @ 60 fps	8 LVDS Output @ ~160fps
Architecture	Interline CCD; Progressive Scan	Exmor CMOS read out technology
Format	1”	1/1.2”
Package	68 pin PGA	118pin - LGA

It is unfortunate that current IMX174 sensor based cameras only comes with USB3.0 interface (Pointgrey). Market adaptation to this interface is still uncommon due to various reasons. Most OEM equipment makers at this moment still prefer the traditional Cameralink technology. 


Existing machine vision lighting and lens technology prove to be sufficient when working with Truesense based cameras. There is no pressing need at this moment to address the sensitivity or the dynamic range issues on existing equipment.

Also, if IMX174 serves only machine vision application it will do no justice to the specification it possess. The wide dynamic range in combination of its HD format, delineate a perfect blueprint for high end outdoor daylight camera.


IMX174 is an excellent product and the landscape for less than 4Mpx industry cameras will definitely be redefined. 

CMOS technology that equals CCD image quality

Traditional machine vision imaging had been dominated by the CCD technology. CMOS was used either in simple inspections or very specialize applications that usually uses rolling shutters.

The CMV4000 image sensor introduced by CMOSIS changes the believe that CMOS sensors are low in dynamic range and noisy. CMV4000 is a highly sensitive global shutter CMOS image sensor with pixel size of 5um and a resolution of 2048 x 2048.

Based on an eight transistor pixel architecture, its state of the art correlated double sampling (CDS) reduces the dark current noise and fixed pattern noise.

It is high speed, with 16LVDS output @ 480Mhz, this equates to about 180 frame per seconds at full frame rate. At about 60dB, its dynamic range tails tightly behind the high end CCD sensor. There is no doubt that CMOSIS have set a new bar for the imaging industry.