Segment Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 28, 2012	Sep. 28, 2012	Jun. 29, 2012	Mar. 30, 2012	Dec. 30, 2011	Sep. 30, 2011	Jul. 01, 2011	Apr. 01, 2011	Dec. 28, 2012	Dec. 30, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Revenues	$ 72,946	$ 73,298	$ 74,484	$ 71,925	$ 67,916	$ 65,951	$ 65,106	$ 73,473	$ 292,653	$ 272,446	$ 248,753
Operating income	13,715	14,728	17,361	11,816	11,193	16,524	13,377	12,366	57,620	53,460	43,241
Capital expenditures									4,942	3,835	2,475
Depreciation and amortization									4,711	4,411	4,323
Engineering and other scientific
Segment Reporting Information [Line Items]
Revenues									213,304	199,772	184,146
Operating income									62,852	57,779	49,182
Capital expenditures									3,264	2,489	1,706
Depreciation and amortization									2,831	2,700	2,730
Environmental and health
Segment Reporting Information [Line Items]
Revenues									79,349	72,674	64,607
Operating income									25,752	22,367	20,801
Capital expenditures									289	313	209
Depreciation and amortization									286	227	195
Operating segments
Segment Reporting Information [Line Items]
Operating income									88,604	80,146	69,983
Capital expenditures									3,553	2,802	1,915
Depreciation and amortization									3,117	2,927	2,925
Corporate
Segment Reporting Information [Line Items]
Operating income									(30,984)	(26,686)	(26,742)
Capital expenditures									1,389	1,033	560
Depreciation and amortization									$ 1,594	$ 1,484	$ 1,398