DLP与LCD投影仪对比

Invited Paper

Comparing digital-light-processing(DLP)and

liquid-crystal-display(LCD)projection technologies for high-quality3D

shape measurement

Chen Gong,Beiwen Li,and Song Zhang*

Department of Mechanical Engineering,Iowa State University,Ames,IA50011.

ABSTRACT

This paper presents a thorough comparison between the digital-light-processing(DLP)technology and liquid-crystal-display(LCD)projection technology on high-quality3D shape measurement.Speci?cally,each individual color channel and the combination of three channels together are studied with the focused sinusoidal pattern(FSP)method and defocused binary pattern(DBP)method.Experimental data indicated that for slow speed measurements,because of its higher contrast, DLP has the advantage for the DBP,or when the precision synchronization is presented for FSP method.Since LCD does not have rigorous timing requirements,it provides more?exibility for system development for FSP method. Keywords:Binary defocusing,liquid crystal display(LCD),digital light processing(DLP),fringe analysis,structured light

1.INTRODUCTION

Digital fringe projection(DFP)techniques have become one of the most popular3D shape measurement techniques due to their speed,accuracy,and?exibility.1,2Conventional DFP systems usually use digital video projectors to project sinusoidal fringe patterns on to the desired object for3D shape measurement.Essentially,there are two main approaches for sinusoidal fringes generation:the focused sinusoidal pattern(FSP)generation method,and the defocused binary pattern (DBP)method.FSP method uses an in-focus projector to project computer generated sinusoidal patterns;and the DBP method uses binary patterns to feed into an out-of-focused projector that produces sinusoidal patterns a result of defocusing.

The FSP method has been widely adopted and extensively developed.However,there are still some limitations.As a nonlinear device,digital video projectors have gamma effect:when fed with ideal sinusoidal patterns,the projection output becomes non-sinusoidal.If not properly corrected,the nonlinear gamma effect will jeopardize the phase quality and the measurement accuracy will be limited.In order to solve this problem,numerous gamma correction methods have been developed and applied with success,3–8nevertheless,they are usually limited to particular system settings(e.g.,camera-projector system setup).Moreover,the nonlinear gamma could change over time and thus frequent calibration is required maintain measurement accuracy.9

DBP method was proposed recently by Lei and Zhang10to overcome the nonlinear gamma effect as well as speed breakthroughs.DBP method only uses two grayscale values(0and255),and sinusoidal patterns are created as a resultant of lens https://www.360docs.net/doc/3513536440.html,paring with the conventional FSP method,DBP method has the speed advantage and is easier to be implemented if a digital light processing(DLP)projection system is used.

Since3D measurement accuracy is highly dependent of phase quality,examining fringe pattern output from digital video projector is crucial.DLP is the most extensively used projection technique nowadays.The key component of the DLP projector is the digital micromirror device(DMD).By tilting the mirrors on and off and use a time integration method, different grayscale values can be generated.11DLP projectors have been extensively used in high-quality,high-speed3D shape measurement.12–15

Another projection technique that has been around for a long time is the liquid crystal display(LCD)projection tech-nology.Unlike the DLP projector which usually has a single chip con?guration,LCD projector employs3chips and therefore it has a stable projection output.Since LCD projectors are more light-ef?cient,it typically produce higher ANSI *song@https://www.360docs.net/doc/3513536440.html,;phone151********;fax151********;https://www.360docs.net/doc/3513536440.html,/?song.

Optical Metrology and Inspection for Industrial Applications III, edited by Sen Han, Toru Yoshizawa, Song Zhang, Proc. of SPIE Vol. 9276, 92760Q · ? 2014 SPIE · CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2071536

lumen outputs than DLP projectors.16LCD projectors have also been used in3D shape measurement?eld as early as the 1970s.17–21

This paper presents a thorough comparison about the performance differences between the DLP and the LCD projection techniques when they are used for3D shape measurement.In this research,we used two commercially available projectors that have similar speci?cations and price.Speci?cally,each individual color channel and the combination of three channels together are studied with the FSP and DBP methods.Experimental data indicated that for slow speed measurements, because of its higher contrast,DLP has the advantage for the DBP,or when the precision synchronization is presented for FSP method.Since LCD does not have rigorous timing requirements,it provides more?exibility for system development for FSP method.sinusoidal method.

Section2explains the principles of DLP and LCD technologies.Section3shows comparing experimental results for both the FSP and the DBP method.Section4summarizes this paper.

2.PRINCIPLE

2.1Fundamentals of DLP and LCD technologies

DLP and LCD are two major projection technologies that are extensively used nowadays.The key component of a DLP projector,the DMD chip,is used to generate different grayscale values(0to255)through time modulation.11Speci?cally, by toggling each mirror either into(ON)or away(OFF)from the optical path and precisely controlling the ratio of ON and OFF time,different grayscale values are generated.For example,to generate a grayscale value of128,the mirror stays ON for approximately50%of the time.For a single chip DLP projector,during one projection cycle,different color channels have to be turned on and off sequentially in order for the projector to generate color images.

The LCD projector generates grayscale values in a completely different manner.The LCD projector uses liquid crys-tal to manage the amount of light that pass through the projector chips:by changing the orientation of the liquid-crystal molecules between two polarizing?lters(parallel and perpendicular),the percentage of light that passes through the polar-izers can be modulated.The orientation of the liquid-crystal molecules cannot be comprehensively controlled,there will be some light leakage that can in?uence the black level of the generated images.Thus,the overall contrast ratio of an LCD projector is typically lower than that of a DLP projector.16

We carried out some simple experiments to to demonstrate the different mechanism of generating grayscale images. In this research,we used a DLP projector(Model:BenQ W770ST)and an LCD projector(Model:Epson730HD)for all experiments.A photodiode sensor(Model:Thorlabs FDS100)was used to monitor the photocurrent that was converted to voltage through a simple electronic circuit.The voltage signal was monitored by an oscilloscope.Figures1shows the measurement results when both projectors was fed with uniform green images with different grayscale values.Figures1(a)-1(d)show the results from DLP projector.These data shows that the DLP projector turns on twice for each projection cycle.The percentage of ON time reduces as the input grayscale value reduces.DLP can generate nearly pure black image because it is based on a re?ective projection technology.

Figures1(e)-1(h)illustrate the results from the LCD projector.In contrast,the G channel of LCD projector is turned ON throughout the whole projection cycle.Thus,the LCD’s output signal is rather stable comparing to the DLP projector. Unlike the DLP,when the input grayscale value is0,there is some light leakage,as shown in Fig1(h),reducing the contrast of the projector.

2.2Three-step phase-shifting algorithm

A three-step phase-shifting algorithm with equal phase shifts is used to analyze the phase.Mathematically,the fringe images used can be represented as,

I1(x,y)=I (x,y)+I (x,y)cos[φ?2π/3],(1)

I2(x,y)=I (x,y)+I (x,y)cos[φ],(2)

I3(x,y)=I (x,y)+I (x,y)cos[φ+2π/3].(3)

(a)

(b)

(c)

(d)

(e)(f)(g)(h)

Figure 1:Example of the projected timing signal when the projector is fed with different images.(a)-(d)DLP projector is fed with a uniform green image with intensity of 255,128,64,and 0,respectively;(e)-(h)LCD projector is fed with a uniform green image with intensity of 255,128,64,and 0,respectively.

Where I (x ,y )is the average intensity,I (x ,y )represents intensity modulation,and φ(x ,y )stands for the phase to be solved for.The phase can be retrieved by simultaneously solving Eq.(1)-(3):

φ(x ,y )=tan ?1√

3(I 1?I 3)

2I 2?I 1?I 3.(4)

Since an arctangent function only gives phase value ranging from ?πto π.A spatial or temporal phase unwrapping algorithm is required to obtain a continuous phase map.The phase unwrapping locates the 2πdiscontinuities and ?x them by adding or subtracting integer number of 2π.Based on the smoothness assumption that the surface geometry is “smooth”,a spatial phase unwrapping algorithm can be adopted.22If the surface geometry is arbitrary,a temporal phase unwrapping algorithm is usually adopted.The temporal phase unwrapping uses additional fringe patterns to ?nd the cues to determine 2πjumps for each pixel.In this paper,the temporal phase unwrapping algorithm discussed in 23was used.

3.EXPERIMENTS

3.1Test system

In this research,we used two projectors,a DLP projector (Model:BenQ W770ST)and an LCD projector (Model:Epson 730HD).The DLP projector has a single 0.6”DMD chip with a native resolution of 1280×720.The projector lens is F/2.60-2.78with a focal length of 10.20-12.24mm.The nominal output light intensity for the projector is 2500Lumens,and its contrast ratio is 13,000:1.The Epson 3-chip LCD projector has a lens of F/1.58-1.72and focal length of 16.90-20.28mm.The projector’s native resolution is 1280×800,the nominal output light intensity is 3000Lumens,and a 12,000:1contrast ratio.

During the experiments,we used the same camera (Model:Jai Pulnix TM-6740CL)to capture the projected images.The camera’s resolution is 640×480with a maximum frame rate of 200fps and the pixel size is 7.4×7.4μm 2.We used a 16mm focal length Mega-pixel lens (Model:Computer M1614-MP)at F/1.4to 16C.To make the comparison relatively fair,the hardware system was set up in a way that the projectors have same amount of focusing,the projected fringes have the same period,and the area captured by the camera is roughly the same.

3.2Timing of projectors

As discussed in Sec.2.1,LCD projectors and DLP projectors generate grayscale images in a completely different manner.

As shown in Fig.1,the speci?c DLP projector we used generates two pulses in a 1/60sec projection cycle for a single color channel (e.g.,green).To investigate how these pulses will affect measurement accuracy,we determined the timing (the time each pulse starts or ends when the projector is projecting a grayscale value of 255for a single color channel)for each color channel during one projection cycle.Since LCD projector does not generate pulse during projection,the timing determined from the DLP projector are shared with the LCD projector to offer a fair comparison.Moreover,we set the camera exposure time to start with the computer’s VSync signal and end on (the middle of the pulse)or off (the middle between two pulses)a DLP’s projection pulse.24For example,when using the G channel,we select four exposure times 3.80,8.00,12.10,and 16.67ms,as illustrated in Fig.2(b).

Exp 1Exp 2Exp 3Exp 5DLP

LCD

Exp 4(a)VSync VSync

Start Exp 1Exp 2Exp 3Exp 4DLP LCD (b)Start

Exp 5Exp 2Exp 3Exp 4Exp 1(c)

Figure 2:Timing chart for camera exposures when only one channel is used.The camera exposure starts at the VSync signal and stops at a different time.(a)Only red projection exposure times (Exp 1,...,Exp 4are 7.60,11.80,15.90and 16.67ms);(b)Only green projection exposure times (Exp 1,...,Exp 4are 5.70,10.00,14.10,15.90and 16.67ms);(c)Only blue projection exposure times (Exp 1,...,Exp 4are 3.50,7.50,11.80,15.00,and 16.67ms);

3.3Comparing experimental results for FSP method

To evaluate the performance of both projectors,we ?rst measured a uniformed white board.We used the exposure times shown in Fig.2for a single color channel capture;and 3.50,5.70,7.50,7.60,10.00,11.80,14.10,15.00,15.90,and 16.67ms when the projector is projecting white images (i.e.R =G =B).The phase map,Φ,is obtained using a three-step phase-shifting algorithm and a temporal phase unwrapping method.Then phase error is determined by comparing against the ideal phase,Φi .The ideal phase is obtained through the use of a nine step phase-shifting method with narrow binary patterns.25That is,phase difference ?Φ=Φ?Φi ,and root-mean-square (rms)of the phase difference (?Φ)was calculated to quantify phase error.

Figure 3shows the comparing results.All data shown in this subsection are captured using a fringe pitch of 18pixels;the nonlinear gamma was also corrected using the method introduced in.7For the ?rst set of experiments,the camera’s aperture is manually adjusted to ensure the captured image with highest possible contrast (i.e.,as bright as possible but not saturated).To ensure fair comparisons,the captured image brightness is visually similar for all fringe patterns,and the captured fringe width is adjusted to be almost identical for both projectors.

These experiment data show that the DLP projector outperforms the LCD projector if the exposure is in full projection cycle regardless of the color channel used.When the exposure time is shorter than a projection cycle,DLP’s performance varies from on pulse to off pulse:on pulse’s performance is worse than off pulse’s performance.In contrast,the phase quality generated by the LCD projector is much more stable within the projection cycle.In general,when single color channel is used and the exposure time is on pulse,the LCD projector generates better quality phase;and when the exposure is off pulse,the DLP projector has a better performance.

When using a white image (i.e.,R =G =B),as shown in Fig.3(d),the DLP’s performance is still not stable when switching between different exposure times.However,the phase quality variation is not as large as that from a single channel.In contrast,the LCD projector always generates relatively stable phase quality regardless of the exposure time used.One may also notice that the DLP projector is consistently better than the LCD projector,which is not the case when a single color channel is used.We believe the difference is that when three color channels are used,the DLP projector has substantial contrast improvement;but the contrast improvement of LCD is not as much as that of DLP due to light leakage.

(a)(b)(c)(d)

Figure3:Comparison between LCD and DLP projectors when camera aperture was manually adjusted and the FSP method was adopted.These?gures show the phase rms errors.(a)Red channel;(b)Green channel;(c)Blue channel;(d)White light(R=G=B).

On pulse Off Pulse

DLP(rad)0.04960.0169

LCD(rad)0.03860.0364

Table1:Phase rms error when only green channel was used with exposure time of5.70ms(on pulse for DLP)and10.00 ms(off pulse for DLP)

From Figs.3(a)-3(c),one may notice that using the DLP’s R channel gives the largest phase error.This is because a single chip DLP projector projects RGB channels sequentially:our DLP projector starts with B channel followed by G and R channel.This means that when capture R channel,the gap between the VSync signal and the Red channel is the largest. This allows more ambient light to enter to the camera,reducing the signal to noise ratio(SNR)and thus phase quality. In contrast,the LCD projector turns on all three channels simultaneously,the difference between different channels is not obvious.

As discussed previously,the?rst set of measurements were performed by manually adjusting the camera aperture. Though visually inspected,the difference in fringe brightness is inevitable,and thus the LCD error curve is not smooth when different exposure times are used,as shown in Fig.3(d).To reduce the in?uence of human inspection,we carried out more experiments when the camera’s aperture remains untouched.Table1shows the results using two short exposure times(Exp1and Exp2shown in Fig.2(b)).The data in this Table shows that,for the LCD projector,the phase error sightly decreases for a longer exposure time due to higher SNR.However,the phase quality are drastically different for the DLP projector because of its on and off pulse exposures,further supporting our previous?ndings.

To visually demonstrate the differences between two projection technologies,a complex3D sculpture was measured.In these experiments,G channel is used along with two exposure times:5.70ms(on pulse time for DLP projector)and10.00 ms(off pulse time for DLP projector).The nonlinear gamma was corrected for both projectors and the temporal phase unwrapping algorithm described in23was used.The sample phase-to-height conversion method introduced in26was also used to convert the unwrapped phase to3D shape.Figure4shows the measurement results.This?gure clearly shows that the LCD projector can consistently provide decent measurement quality with different exposure times.The DLP projector, on the other hand,fails to provide reasonable quality result when on the pulse exposure time is used,horizontal stripes shown in Fig.4(a),although it performs quite well when the exposure is off the pulse.

3.4Comparing experimental results for DBP method

Similar experiments were carried out for the DBP method.Figure5shows the measurement results when measuring the same uniform white board used in Subsection3.3.The exposure times used are the same as the previous subsection as well.In all experiments,both projectors were properly defocused so that high-quality sinusoidal patterns were generated. These experimental results show that the DLP projector always outperforms LCD projector regardless of color channel or

(a)

(b)

(c)(d)

Figure 4:Complex 3D object measurement results using the FSP method when the camera is synchronized with the projector.(a)DLP projector on pulse (5.70ms);(b)DLP projector off pulse (10.00ms);(c)LCD projector on-pulse (5.70ms);(d)LCD projector off pulse (10.00ms)

On pulse Off Pulse

DLP (rad)0.02550.0267LCD (rad)0.04300.0402

Table 2:Phase rms error when only green channel was used with exposure time of 3.80ms (on pulse for DLP)and 8.00ms (off pulse for DLP).

exposure time used.Different from the FSP method,the on/off pulse in?uence on the DBP method is not obvious.This is because when using the DBP method,only grayscale value of 255and 0are used,meaning that the pixel is either always on or always https://www.360docs.net/doc/3513536440.html,paring with the FSP method,this stable projector pixel status of the DBP method gives much better results when a exposure time is short;and when the exposure time is long it provides similar quality of data.In contrast,for the LCD projector,both FSP and DBP methods offers similar quality of data,albeit the FSP method is slightly better if the exposure time is long.This is probably because the contrast is slightly higher for the FSP method.

10

00.050.10.150.20.25Exposure time (ms)

P h a s e r m s e r r o r (r a d )

DLP LCD

(a)0

10

00.050.10.150.2

0.25Exposure time (ms)

P h a s e r m s e r r o r (r a d )

DLP LCD

(b)0

10

00.050.10.150.2

0.25Exposure time (ms)

P h a s e r m s e r r o r (r a d )

DLP LCD

(c)0

10

00.050.10.150.20.25Exposure time (ms)

P h a s e r m s e r r o r (r a d )

DLP LCD

(d)

Figure 5:Comparison between LCD and DLP projectors when camera aperture was manually adjusted and the DBP method was adopted.These ?gures show the phase rms errors.(a)Red channel;(b)Green channel;(c)Blue channel;(d)White light (R =G =B).

Similarly to Subsection 3.3,to reduce the uncertainty introduced by manual adjustment,Table 2shows the results without any manual adjustments.Both projectors perform slightly better when a longer exposure time is used.It should be noted that no drastic difference for DLP projector regardless the use of an on and off pulse exposure,which is consistent to experimental results shown in Fig.5.

The same 3D sculpture was also measured using the DBP method.Figure 6shows the measurement results.Green light and the same two exposure times,5.70ms and 10.00ms,were used;and both projectors were properly defocused.These results clearly show that the results from using the DLP projector is consistently of high quality.However,the result

from using the LCD projector has some high-frequency noise,which could be a resultant of lower contrast or different pixel sizes.

(a)(b)(c)(d)

Figure6:Complex3D object measurement results using the DBP method when the camera is synchronized with the projector.(a)DLP projector on pulse(5.70ms);(b)DLP projector off pulse(10.00ms);(c)LCD projector on-pulse(5.70 ms);(d)LCD projector off pulse(10.00ms)

4.CONCLUSIONS

This paper provided a thorough comparison of the performance between the DLP projector and the LCD projector when they used for3D shape measurement.Due to the completely different mechanism of grayscale image generation,for the conventional FSP method,the LCD projectors have more?exible exposure choices whereas DLP projectors have a stringent timing requirement;when this requirement is ful?lled,DLP can provide higher quality3D measurement results. When used with DBP method,bene?ting from its higher contrast ratio,the DLP projector proves to be a better choice.

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