FOR ENERGY EFFICIENCY,

STREET LIGHTING AND SCALED PHOTOMETRY Prof. S A Waheed Raye

Introduction:Frequent accidents are well known facts on the roads poorly illuminated and maintained.

About a year back or so, a lady was knocked down during night on EEH (Mumbai) and her body was repeated crushed by other vehicles till the day light made it visible and gave relief to the dead body. The looser, in terms of life and safety, is the innocent public. This is the news that compelled me to start writing this article on street lighting and lantern.

Having conducted street lighting design, Lantern design and street lighting surveys for last 35 years throughout the length and breadth of India, I feel great need of Lanterns users’/selectors’ education. Author has taken up this cause for education and guidance of users (namely maintaining installing bodies) at no cost to them anywhere in the country. For, I have firm belief in the verse “Son of man did not come to be served but to serve and give his life as ransom for many”. (Mrk 10:45)

Street lighting enjoys the major share of energy consuming lighting load in our country. Still, it is the most neglected part from the energy efficiency point of view. There are two major sequences of this subject concerning energy efficiency. Firstly, it is the selection of street lanterns and secondly it is the maintenance of their installation. These two aspects can contribute in a much bigger way to the cause of energy & life saving for our country if practiced correctly. These aspects are the part of this article to bring home the truth.

Preamble:The most energy wasteful product of illumination engineering in our country is the street

lantern. Here also we find two major limitation of our industry. It is, Firstly, the lack of technical knowledge of small scale manufacturer. Consequently even multinational companies have limitation related to quality consistency of street lanterns, which originate from the small scale venders’ limitations.

Thus, in given situation, the performance of lantern can not be guaranteed at any level at all. This inconsistency gets further compounded due to the lack of training to the user by supplier with regards to the lamp and luminaire compatibility and maintenance procedures.

Secondly, having no means to verify the technical claim of suppliers, the user goes on deciding tenders of street lanterns on the basis of price and watts only. Thus the real time technical data or their evaluation carries no weight age at all.

Eventually, a commercial man with management skill running small scale manufacturing unit is happiest person in this exploitive situation. But, the scaled photometry can now help not only such buyers but also manufacturer to evaluate energy related photometry.

Scaled Photometry in a nut shell:The scaled photometry is simple but accurate way of evaluating street lantern without

spending huge amount on the testing instruments. This will prove to be very much helpful in providing simple and quick means of testing the photometry of street lanterns, which is based on the author’s long experience in street lighting, lantern designs and surveys conducted.

This photometric method is coupled with suitable software which can also give you every photometric parameter of street lantern such as spread, throw and SLI value. In addition to this it evaluates lantern from the application point of view. Thus it generates Iso-lux, Iso-candela and other associated information for any given width of road. In other word this method is combination of photometric testing and application design combine together.

Scaled Photometric Measurements:In case of scaled photometry the laboratory is modeled to suit quick adjustments and

instant measurement. This also means that you also get instant comparative results, thereby proving helpful in deciding in favor of an efficient brand supplier. Such a laboratory consists of an artificial road surface imbedded with sensors. Each sensor is screened by suitable baffle to get best accuracy. The road surface texture can also be varied to study luminance values as well.

The suitably scaled height pole and grids sizes are chosen to make precise measurement in sufficiently large field. The reference lamps and reference ballasts are used to get reproducible output. Only one lantern is mounted on the pole.

The outputs of this photometry are the direct application results, In terms of either national or international standards. You can also plot iso-candela, Isolux diagram along with the average illuminance at different mounting heights and width of road for group of lanterns. The outcome may also be part of comparative study of lanterns so as to find better field performing product.

Maintenance; Relamping and Energy Wastage:-It is unfortunate that now-a-days the marketing giants do not hold training program for

the benefit of users. Such programs are important from the energy efficiency point of view. These programs make user aware about compatibility of lamp and luminaire. They also provide opportunity to user to know, how not to or, how to maintain the street lantern.

Lake of training and education leads to many misuses of lantern. The most abusive situation for street lanterns, I have come across, is the coated elliptical lamp being used in a lantern which is originally designed for tubular clear lamp. User seems to be unaware about such misuse of lantern. For him 250watt lantern is suitable for 250watt lamp whatever may be the type. In order to see, what happens when such lanterns are misused, refers to the photo 2 to see how patchy Luminance becomes. A polar diagram is also being shown here (Fig No.1) with account of the consequence given thereafter.

In such situations (see Photo-2) deriver can see any obstacle only when enclosed by bright patch. His eye gets adapted to that luminance. Therefore , he cannot see what is happening on rest of the road.

Photo-1Tubular lamp’s street light with suitable lamp

The diagram shown in Fig No.1 relates to photo 4 and pertains to a street lantern designed for clear tubular lamp of 250watts HPSV which is show by photo-1. The obstructing diameter of this clear tubular lamp is only 10mm (Photo 1). Thus obstruction being small the size of reflector was reduced to 240mm radial opening. Therefore design obstruction ratio comes to 1/24 times in this case. Polar intensity distribution, for this correct combination is shown in red color in (Fig No. 1) polargraph.

Photo 2.Misuse of lantern by re-lamping with coated elliptical lamp.

Figure No.1: Polar intensity distribution of street lantern for tubular lamp

(Blue curve shows elliptical coated lamp being used)

The output of misuse installation is shown in photo 2 when an elliptical coated lamp (ie Photo-3) of same wattage was used in the above lantern the obstruction increased to 1/3. For the obstructing diameter of these coated lamps is 82mm.Thus operating this lantern with coated elliptical lamp increases the obstruction

Photo 3:Misused lantern being used with coated elliptical lamp

ratio to 1/3. By a simple logic we loose more than 33% lamp lumen output due to this obstruction. Whereas, the lantern was designed to obstruct just around 4% lumen output of lamp. Polar intensity distribution of such incorrect combination is shown by blue line graph (Fig No. 1). The consequence of such misuse is explained below.

Average of illuminance on road surface, with coated elliptical lamp in the street lantern, had fallen to 12 Lux with the uniformity ratio falling to a dangerous level of 0.1(Fig 2b). The same lantern when used with tubular lamp gives an average of 25 Lux on road surface with uniformity being more than 0.4(Fig 2a). These results pertain to the same road width, mounting

height, tilt and overhang. The loss of lumen output is almost 50%. But the worst effected parameter is uniformity ratio of course.

Fig 2a.Iso-lux line of original designed with correct lamp.

Photo 4Actual output of correctly used tubular Lantern.

Fig 2b:Iso-lux lines of lantern when misused with elliptical coated lamp.

There is inverse kind of misuse of lamp; when a tubular lamp is used in street lantern which is designed for coated elliptical lamp. In this case the loss of lumen output is not appreciable but the intensity distribution is distorted to such an extent that it brings down the uniformity on road surface to a dangerously low level. The resultant polar curves are shown below (Figure No.3).

Figure No.3Polar distribution diagram for lantern when misused inversely

(i.e. misusing tubular lamp in lantern designed for coated E-lamp)(Blue curve shows resultant distribution)

Maintenance; cleaning Lantern and Energy wastage:-The benefit of cleaning different component of lantern can not be denied. I am not

writing this because you do not know it, but because you do know it. The fact, which is not known, is the tangible benefit of such cleaning. Do not be surprised my friends if I say that on some of the roads in Mumbai the benefit will come to be equivalent to 50% of operating cost.

For example let us say a particular road’s stretch of a road is being cleaned every 9 months. The up gradation, when cleaned every 6 month, in illumination can be found from the following curves “A” and “B” (Fig 4). Same lantern when cleaned every six month will be having almost 8.5 % more lumen maintenance. This translates to 91.5 % of operating cost.

However, practically the lanterns are cleaned at intervals of more than one year. In fact the saving in such cases comes out to be even more than 50% of operating cost. In reality some of the roads are so polluted that even cleaning every 6 months will become more beneficial than 50% of operating cost. However, since the street lighting billing is based on number of lanterns and not actual energy put to social use, which related to users’ safety, it goes unnoticed.

Question arises, who pays for this inefficiency of incorrect maintenance and misuse of street lanterns. Of course it is innocent public who pays this cost by endangering their lives and even losing it many a times. But does the government agency or street lighting authority saves this much fund? No, not at all, it is just lost into blues at the cost of public life.

For example once on eastern express highway in 80s’ we have taken estimate of dust and dirt factor. And found surprising results. The Consequence of such negligence, are too serious accidents. Frequent accidents are well known facts on such roads as mention in the beginning

Figure No. 4Lanterns’ cleaning frequency 9 months verses 6 months and improvement

In average illuminance

Street Lighting Designs Specifications:The Bureau of Indian Standards has published various standards to cover the subject of

Lantern and it application. The standards are in line with the true requirements

---------------------------------------------------------------------------------------------------Classes of road Illumination Areal Uniformity

Average Lux on surface area -------------------------------------------------------------------------------------------------

A1 30 . 0.4

A2 15 0.4

B1 8 0.3

B2 4 0.3(Table 1)

of street lantern, but they are too incomprehensive, to be used by general supervisory staff at Municipal Corporation or town planers' offices. For example the application standard classifies the road into four classes based on the traffic density and traffic speed. These information are given above (Table 1).

In the cities in actual practices the illuminance has now been increased a lot. For example in big cities such as metropolis like Mumbai and Delhi Lux values on some of class A1 road (when right lantern is used) will be close to double the values specified by standards. This is because the traffic density has changed, which was the consequence of economic boom. Secondly these values, given in IS, are minimum average for given class, and there is no limit for maximum average for these classes of road.

Specification of luminance valuesFunctionally the road surface play very crucial role in performing the task visibility for

safety of users. The measurement of such visibility involves measurement of luminance. These measurements are time consuming and therefore normally avoided in field, unless the measurements are done by automated method.

However, the energy consumption verses luminous function can be well balanced by correct selection of lantern by testing of illuminance in lux. Therefore illuminance and its uniformity are the parameter specified by earlier specification, which suffice for evaluating street lantern performance in field irrespective of road surface characteristics.

But, to take care of over all safety aspect, road surface characteristics also play a very important role. In that case luminance of road surface in term of candela per square centimeter is specified by standard. However in order not to confuse reader we stick to simple term like lux, which is function of only street lanterns’ photometry, and is measurable directly. More so, because this parameter in street lighting, depends solely on the maintenance schedule and procedures adapted and is independent of road characteristics. Therefore user has to deal with Lux as measurable term than any other term on day to day basis.

Luminance verses Illuminance:Latest Indian Standard specification of street lighting specifies Luminance value of 0.5

Cd/sq.m , 1 Cd/sq.m , 1.5 Cd/sq.m and 2 Cd/sq.m for B2,B1,A2 and A1 classes of roads respectively However, as stated earlier the measurement of Luminance is more tedious and time consuming (unless automated) than illuminance. Since, Luminance also takes into account the road surface reflectivity, which is independent of street lantern photometry. Therefore, it is the measurement of Intensities in Candela and illuminance in Lux which suffice to be evaluated for the street lanterns and their effectiveness.

Scaled photometry can also evaluate luminance values of road surface with addition of road surfaces flooring. However this will lead to the necessity of reference road surface. Consequently the testing system cost shall go up. For all practical purposes scaled photometry

related to intensity and illuminance is sufficient for efficient decision making. Thus we need not necessarily build luminance scaled measuring facility. But it should be remembered that luminance is closer to expressing visibility of objects or obstacles. But for evaluating street lanterns performance the Illumnance measurements suffice.

Street Lantern Designs Specifications:From the energy efficiency point of view it is necessary to educate the selector of the lighting system to be careful from un-informed suppliers. Those suppliers copy the product and it's technical literature and submit to the tendering customers. Unfortunate, the buyer has no field evaluation method. But, with the advent of computer and scaled photometry evaluation becomes very easy.

Photo-5 Spread and throw being shown in actual installation.

(White colour lines show throw along the length of road, and red colour lines show plane of the spread across the road.)

For street lantern designs, there are specifications related to intensity distribution of the lanterns. They are not studied by many suppliers because they can’t afford costly photometric measurement systems. These specifications which are neither apparently------------------------------------------------------------------------------------------------------------

Throw : Spread : ControlType degrees : type degrees : Type Value===============================================================Short Γpeak < 60 : Narrow Γ90 < 45 : LimitedSLI<1---------------------------------- :---------------------------------------- : -------------------------

70 : 45 : 1=<Intermediate =< Γpeak =< : Average <=Γ90 <= : Moderate SLI

60 : 55 : =<4------------------------------------:---------------------------------- :-----------------------Long Peak > 70 : Broad Γ90 > 55 : Tight SLI>4

(Table 2)

useful nor comprehensive to the users, and also apparently do not have much utility, except to prove the sophistication of our standard. However, technically they do provide support data to a designer to compare different lantern designs. These specifications are given in above table 2. These characteristics like Throw and Spread are specified in terms of degrees of angle Gama ( Γ ) with a subscript, telling us in which C plane (i.e. vertical plane) the gama is measured. For example Γpeak means angle in vertical plane containing peak intensity. But Γ90 means angle in vertical plane which is 90 degree to road axis (Photo No. 5). All these planes pass through the vertical axis of the lantern when mounted at 0 degree tilt.

The Trick of making technical Data:Readymade photometric data of a multinational company and other parameters are freely

available in market. They are, therefore normally copied by any small scale manufacturer. Subsequently they are claimed to be their lantern data. Unfortunately even some of so called multinational have their own methods to arrive at those data published by them. Thus, their claim may also not be true some times.

Therefore, the performance of lantern may not match the submitted datasheet. In other words the claims of the supplier should be verified for the sake of energy efficiency. The lantern shall be put to test to see that it performs well in field and avoid energy wastage.

CONCLUSION:-Function of Lantern:Very important functional aspect of lantern is to see that the road will be well illuminated

quantitatively as well as qualitatively. The illuminance values provide information about lantern performance independent of road surface. Therefore, average illuminance from lanterns shall be compared to find the better performing lantern. Thus we can select an energy efficient lantern in this manner by quantity of output, but the quality aspect shall be as follows.

Average illuminance (the quantity) will not be able to show better performing lantern unless uniformity (the quality) of illuminance is also compared. These to parameter of street lantern need to be evaluated for different width of road with suitable spacing to arrive at comprehensive results and eventually conclude. It may happen that a particular lantern may be suitable up to a particular width of road thereafter it becomes uneconomical. . .

With the advent of computer it has become easier to see street lanterns’ performance at varied heights, angles of tilt and widths of the road. The software can run through billions of calculation and eventually inform the purchaser about long term implication of a lantern. For example one can find out the "running cost”, "maintenance cost" and the "life cycle cost" of each lantern. This is in addition to the verification of spread, throw, SLI, iso-lux, iso-candela and average illuminance/uniformity on road surface.